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Lean Code’s P0171 and P0174 – Common Causes

10:47 am DIY, Ford Problems, How To Auto Repair, Lean Codes

Ford Expedition lean codes could caused by a leaky PCV hose.

If a car has a check engine light on and has P0171 and P0174 codes stored, the most likely cause is a vacuum leak. These codes are set by the car’s computer when it sees too much oxygen in the exhaust. Excessive oxygen readings in the exhaust will set lean codes. This does not mean that the oxygen sensors are defective. It is a common mistake for people to replace oxygen sensors when lean codes are present. Just because the code is related to oxygen sensor readings, it doesn’t mean that the oxygen sensors are bad. If both of these codes are stored this means that both banks of the engine have excessive oxygen in the exhaust. If the oxygen sensors are replaced it’s most likely that the same codes will reappear. It is very uncommon for two oxygen sensors to fail at the same time.


Therefore the most likely cause of P0171 and P0174 codes is a vacuum leak. The source of a vacuum leak can be one of many things. The most common cause of a vacuum leak is a leaky hose. Vacuum hoses over time can become dry and brittle. The rubber deteriorates and can leak causing a vacuum loss. Many times a leaky vacuum hose can be found by listening for a hissing sound. A large vacuum leak can even make the car run rough at idle. In some cases it will not idle without feathering the gas pedal. Another way to check for a vacuum leak is by using a propane bottle with the hose attachment. By introducing propane to the area where the suspected leak is you can sometimes notice a change in the idle. This is particularly true with cars that don’t have computers. Cars that have computers can quickly compensate for the change in fuel mixture. This makes checking for vacuum leaks in this manner more difficult. If you have access to a scan tool, checking the oxygen sensor readings or fuel trim can be done while checking for leaks with propane. If the readings increase when you are adding propane to be suspected leak area, this indicates a vacuum leak. Another way to check for vacuum leaks is with a smoke machine. Evaporative smoke testers can be used to check for vacuum leaks as well. Evaporative leaks have to do with fuel vapors and a smoke machine is used to find those leaks. But evaporative smoke machines can also be used to find engine vacuum leaks easily. To check for engine vacuum leaks with a smoke machine, just find a vacuum hose that goes to the engine and attach the hose on the machine. When the smoke machine is turned on, smoke will enter the engine and help reveal the vacuum leak source. It is common for some smoke to come out out certain areas without indicating a vacuum leak, so if you think you’ve found a leak with the smoke is best to double-check it to make sure. The smoke machine is a great time-saver and can help pinpoint vacuum leaks quickly.  

Mass Air Flow (MAF)Another cause of P0171 and P0174 codes can be from a damaged mass airflow sensor. If the reading from the mass airflow is incorrect, the computer can be doing its calculations with wrong numbers. The computer bases the fuel mixture partially based on the amount of incoming air. If the computer sees more air entering the engine than what is actually going in it will enrich the mixture. Therefore if the computer sees less air than what is actually going into the engine it will decrease the amount of fuel in the mixture. Mass airflow sensors can be damaged by dirt getting past the air filter and sometimes from oil that can come off of aftermarket air filters such as K&N. K&N air filters have to be lightly oiled and if excessive oil is applied some of it can come off and endup in the mass airflow sensor. If oil contaminates the sensor it can cause skewed readings.  

Fuel Pressure Incorrect fuel pressure from the fuel pump can also rarely cause a lean condition. The fuel pump regulator could be allowing the wrong amount of pressure to be pumped to the fuel injectors. If a fuel filter is clogged or restricted this can alter the amount of fuel that reaches the injectors also. Fuel pressure is not likely to cause lean conditions though, due to the computer’s ability quickly compensate and adjust the air fuel mixture.

Some comments from readers.

We recently had a check engine light come on and after having it scanned at a local parts store we replaced an oxygen sensor. The parts store employee had recommended the oxygen sensor replacement however the code quickly returned. After going back to the parts store they recommended a fuel filter. After buying a fuel filter and going home my husband was about to install it when I searched online and found your article. After explaining to my husband that the cause could be from a leaky vacuum hose, he ran the car with the hood up and we could hear hissing. The problem was exactly what you said, an inexpensive rubber hose. Thanks so much for sharing this auto repair tip. Although the parts store employee was trying to be helpful he was recommending replacing parts that were not bad. I don’t think he was doing this intentionally, but it was starting to cost us an arm and a leg! Thanks again for all your help.

 

Here’s our experience, my wife’s 2003 Ford Expedition was stalling out while driving slowly and at stop lights, etc…  We took it to an Advanced Auto parts store and they scanned it with a code reader and said it was the bad coil and spark plug. We changed them but did not get any better. We thought maybe we had a defective part, so they exchanged them and we installed the parts again to no avail, it was still stalling. We took it in again yesterday the parts store said it was the fuel filter. We took the car to the Ford garage and had that changed, but it was still not running right. They did a diagnostic test and came up with the PO171 and PO174 codes again. I searched those codes on the Internet and found your site and read automotive repair blog. My wife read that there might be a leak in a vacuum hose. She reminded me that she had heard hissing noises over a week prior and had mentioned it then. I didn’t listen, forgot about it until she said again, “I told you I heard hissing sounds a while ago!” So I checked the hoses and found that there is a leak at an elbow in one of the hoses! No telling how long this would have gone on, but with your help we were able to repair the problem ourselves and not continue throwing money at the problem. Thanks for your help and I will recommend your site to everyone I know!

278 Responses
  1. Mike :

    Date: August 9, 2009 @ 5:43 pm

    I have a 2001 Jaguar S-Type. It was throwing a code P0171 as well as P0174. We took it to the mechanic and they said it could be a number of problems, so I researched online and found your site. It was the Intake hose on the TOP of the Motor. I could Hear “Hissing” and it was very easy to replace. The new hose only Cost me $6
    Thank you very much for your help! You saved me alot of money~

  2. sprawlking :

    Date: April 22, 2010 @ 10:22 pm

    I’m writing to hopefully provide solace for those who are perplexed by these two bewildering codes. I recently had an issue with these two codes occurring together. From the outset, I paid for a diagnosis at the service dept of my local Ford dealership. They failed to make a diagnosis. I made it clear to them that I had no intention of throwing thousands of dollars at every symptom they could come up with (…and belive me there are many variables); but apparently this had been their plan. It’s inconceivable that the mechanics would have had no experience in dealing with other distressed Explorer owners–enough to know how to resolve the issue without expensive “guesses”. Once I paid for a diagnosis I was determined to hold their feet to the fire–showing up at the service dept everytime the light came back on. Eventually, I gave up and begin the arduous process of elimination. I had spent approximately $300-350 replacing the (mas airflow sensor, Throttle position sensor, and the oxygen sensor). After replacing all these, the light immediately came back on. Forutnately, I took it to a trustworthy mechanic who did a thorough investigation (before my very eyes), before rendering an opinion. He said that the problem was most likely the (intake-manifold gasket). I paid him 225.00 for the diagnosis and to replace the gasket and I’ve been worry-free ever since. The point I am trying to make is for anyone distressed by this should first consider this before spending hundreds of dollars needlessly in replacing all the sensors. Undoubtedly, it’s a money making scheme–dreamed up by the engineers at Ford.

  3. dennisb - Auto Tool Sales :

    Date: April 23, 2010 @ 6:26 am

    Sprawlking, thanks for the comment, I appreciate you taking the time to vent… I mean share your experience :). Intake gaskets (actually upper plenum gaskets) on most Ford Explorers are similar to rubber o-rings. They are a common problem that many times can cause vacuum leaks on 4.0′s. Sometimes they only leak when cold, making them particularly hard to detect by the time the vehicle has been driven to the shop and reach normal operating temperature. This would possibly explain why an Explorer with this problem may run rough or at an excessivley high idle until it warms up.

  4. Hoon :

    Date: September 13, 2010 @ 9:25 am

    i have an issue. had a vacume leak for some time, and now my truck has random delays in starting.

  5. dennisb - Auto Tool Sales :

    Date: September 13, 2010 @ 10:16 am

    Could be related if the problem occurred immediately after the vacuum leak was repaired. It sometimes takes a little while for the computer to put it’s fuel/mixture strategy back into place after it the air intake is back to normal. It’s also possible that the problem you are now having may not be related at all. I would try cleaning the throttle plates and if the problem persists and there are NO codes stored – check for fuel pressure bleed down. Some pressure should remain in the fuel rail, 0PSI in the rail when a start is attempted, will cause a delay in the engine starting.

  6. Larryh :

    Date: December 12, 2010 @ 10:51 pm

    My 2000 ford F150 is showing only the PO171 code by it’s self. Could it still be a vacuum leak or would you suspect an 02 sensor?

  7. dennisb - Auto Tool Sales :

    Date: December 13, 2010 @ 8:14 pm

    Larry, I would look for a vacuum leak. However, lean on both sides would be more of an indication of a LARGE vacuum leak. Good Luck

  8. joe :

    Date: April 4, 2011 @ 5:15 pm

    03 jag x type. p0171 & p0174 codes set. won’t idle cold. misses.

  9. dennisb - Auto Tool Sales :

    Date: April 4, 2011 @ 6:05 pm

    Joe, sounds like a vacuum leak. that’s the first thing I would check. Good Luck

  10. Mike G :

    Date: April 12, 2011 @ 12:10 pm

    I have an 05 saturn vue 2.2 l. And it runs rough some times and. It brings up 171 and 172 codes. Not sure where to start thanks

  11. Jeff :

    Date: April 23, 2011 @ 5:31 pm

    I have a 1998 ford f150 5.4 liter with the codes p1131,p1151,p0171,p0174,p0156 as well as a p0113 code. It barely idles and often dies when I come to a stop unless I push on the gas pedal while pushing the brake. When I spray starting fluid towards the back of the motor, It revs up. So this tells me it’s a vacuum leak, But since the fumes spread out, I’m not sure how to pinpoint the exact leak. Any ideas?

  12. dennisb - Auto Tool Sales :

    Date: April 24, 2011 @ 8:32 am

    Jeff, You could use a propane bottle and a small rubber hose to more precisely check certain smaller areas. However, a common leak on your motor is the PCV hose. It could be a collapsed elbow, could be leaking or both.

    Sometimes you can even hear vacuum leaks on these trucks. Look for collapsed hoses coming from passenger side rear of manifold going to PCV valve. This is by far the most likely cause of the leak. Another possibility, although less common would be intake gaskets. Look for simpler, easier fixes first. Good Luck.

  13. Jeff :

    Date: April 26, 2011 @ 11:33 am

    I just found the vacuum leak. The small rubber elbow at the end of the pcv valve that connects into the intake under the idle air control valve had a huge hole in it. It’s hard to believe that a $10.00 dollar rubber elbow caused so many codes to be thrown. But for now, It seems to be running better than ever. Thanks for the help.

  14. ron schroeder :

    Date: June 29, 2011 @ 12:41 am

    Hi dennisb I have a friends 98 mercury mystique 2.5L v6 with p0171 p0133 p1151 & p1131 trouble codes my buddys snap on scanner said p1131 mass air flow sensor out of range, do u think this could be causing all the codes for bank 1 & 2 & if so should I try cleaning it or replacing it. also gonna check for vac leaks, also thinking about cleaning iac valve, egr & throttle body, etc. just wanted your input. this is a friends car & im trying to help him pass emmissions without breaking the bank, which with high miles I know may be hard. thanks ron.

  15. dennisb - Auto Tool Sales :

    Date: June 29, 2011 @ 6:49 am

    Ron, since you are in the repair business (selling tires) you may be aware of what most mechanics call the Mystique – a “MISTAKE”. These cars had a horrible problem with the fuel injector harness insulation. Be sure not to move any of that wiring if it can be avoided. I had an experience with one of these when inspecting for resale at a car sales supercenter a decade ago. There were so many problems with the car, I wanted it to be auctioned off for wholesale instead of repaired and sold for retail on the front lot. The buyers, for whatever reason, did not listen. When the engine compartment was pressure washed during the detail process, the crumbling, brittle plastic coating (or insulation) was blasted away from the wiring. The car would no longer run and had to be pushed out of the detailing area. A new FI harness at that time was only available in a complete wiring harness for the engine and was going to cost around $1500. Instead, we ended up removing the intake manifold and using black electrical tape on the entire FI harness. The car was then placed in a wholesale auction like I had originally recommended!

    OK, now that I’ve vented – back to your issue at hand… I would check for vacuum leaks first and foremost. Cleaning the IAC would certainly be a good idea, but probably wouldn’t have any effect on the codes set. A dirty MAF (MASS AIR FLOW) sensor could set a lean code possibly, so you could carefully clean that. Also check motor mounts to insure that too much engine travel isn’t pulling on the air intake hose from the MAF to Throttle. A leak past the MAF will allow unmetered air which could result in a lean code. And it would be out of range due to the leak as well. If still setting codes after fixing the above possible issues, there could be Oxygen sensor problems – see our P1131 related article for more ideas to help with diagnosing. Good Luck.

  16. ron schroeder :

    Date: June 29, 2011 @ 3:49 pm

    ok well I cleaned maf & trottle body, gonna change oil too & plugs n wires since wire appear to be arching but I know thats not causing these problems. I read that I can try unpluging maf & drive car to see if codes come back or not. I also seen one hose on large rubber boot kinda loose so I pushed back in. I also did hear hissing in driver back of engine & it appears to be sometype of hose going from lower intake I believe going to egr soneiod I think? (passenger side of engine) but it has a sprial tubing type thing over it & it appears to have fuel line like fittings on both ends. unless it is a fuel line & im just hearing fuel running through it. I tried spraying with carb cleaner but didn’t seem to change much even after unplugging maf. I also hear possible coolant temp sensor or possible crank or cam sensor (cam timing) I even hear worn engine? too car has about 125K well thanks for your input on this POS LOL

  17. ron schroeder :

    Date: June 29, 2011 @ 3:55 pm

    o yea one more thing, even tho changing both upstream o2 sensors with the miles, good maintence. I hear it is highly unlikey that both would go bad & cause these codes to pop up, I cleared codes after cleaning maf & throttle body. Plus I even cleaned upper intake with seafoam by running it through vac line that feeds all cylinders, to see if its sticking injectors or such & put lucas fuel treatment in gas tank & might even try running some shell v power through. well thanks for your time & have a great day, gotta check out some of your tools too sometime lol if wife will let me spend the money. =)’

  18. Doug L :

    Date: July 16, 2011 @ 9:25 am

    I have a 2002 Saturn Vue with a V-6 and AWD. The car randomly does not start and then miraculously does. Sometimes right away, other times after a few hours. I did have a code P0174 but am not so sure this could cause my problem totally. Any thoughts or ideas?

  19. dennisb - Auto Tool Sales :

    Date: July 16, 2011 @ 2:55 pm

    When it’s not starting I would do a fuel pressure test. Could be a fuel pump failing. Good Luck.

  20. george :

    Date: August 18, 2011 @ 9:33 pm

    My 2002 f150 4.2 later keeps having a hissing noise after I changed the isolator bolts and intake plenum gaskets as the Ford tsb bulletin described.I have checked for vacuum leaks on all hoses with carb cleaner and no change.I still keep hearing the hissing coming from the middle of the engine or where the intake plenum is. at this point I have no idea what to do anymore. The check engine light has not come back on after 200 miles but it still wants to stall at idle and going over bumps in the road in 1st and 2nd gear.any ideas what it is?

  21. Gary Greesimunkee :

    Date: August 29, 2011 @ 4:46 pm

    Those 2 codes showed up and I heard the hissing. I couldn’t tell you where in the heck the oil goes… but I own that leaky vacuum hose problem. FIrst thing that’s ever been fixed on my car- where nobody went “prison style” on me after being fixed.

  22. dennisb - Auto Tool Sales :

    Date: August 29, 2011 @ 6:01 pm

    Gary, you may even gain a side benefit of “less oil consumption” after fixing the vacuum leak. Without the huge vacuum leak, the PCV system will work properly and the engine may burn less oil now.

  23. Yu Fu :

    Date: September 27, 2011 @ 11:22 am

    Hi. My 2003 Ford Taurus had Service Engine Soon lamp on and showed the code P1131 lack of heated oxygen sensor, bank 1 sensor 1, 1)switch indicates lean. P1151 heated oxygen sensor, bank 2 sensor 1, switch point indicates lean. P0171 bank 1 system too lean. P0174 bank 2 system too lean. Any ideas what shall I do with it ? Thanks.

  24. owen scott :

    Date: October 13, 2011 @ 12:35 am

    got the codes on my 06 fusion v-6. MAF is good. no vacuum leaks fuel pressure at 35psi. ford says 48 to 70 psi. code is intermittent. usually at low idle or low rpm. any help.

  25. dennisb - Auto Tool Sales :

    Date: October 13, 2011 @ 6:42 am

    Owen, although rare, low fuel pressure can cause a lean code. Your reading is below specification, so there is a problem there that should be addressed regardless if it is causing the lean codes.

    Vacuum leaks are sometimes hard to find because they are small enough to NOT seem to affect the quality of idle or performance. Also, vacuum leaks may only occur when the engine is cold and first started up. After the engine warms, due to expansion, the seals may fit tighter and there’s no leak. That’s why it may be worth smoke checking the engine when it’s cold. A common leak on your engine would be the intake gaskets.

    It’s always a good idea to check for TSB’s (Technical Service Bulletins). Sometimes computer reprogramming or “flashing” is done to tweak parameters slightly. This is done when the manufacturer recognizes there’s really no mechanical or sensor failure, but codes are set due to certain conditions that they did not account for. That’s why it’s always best to check for a related TSB when there seems to be nothing wrong when checking the basics. Good Luck.

  26. kita :

    Date: October 18, 2011 @ 5:28 pm

    I have a 96 bonneville and we put it on a scanner and it stated po171 and something about random misfire..and of course when I went to emissions I failed trying to get it registered. Now my question to you should I waste my time and money with this car because I only paid 500 for it and put about 600 into it already…I’m just trying to way the odds.thanx

  27. dennisb - Auto Tool Sales :

    Date: October 18, 2011 @ 5:37 pm

    Kita, those codes could cost a few dollars to fix or it could be several thousand dollars. if I were you, I would get an estimate for repair and make sure they know the purpose is to pass emissions. After you know what the cost is you can decide. Good Luck.

  28. kita :

    Date: October 18, 2011 @ 6:08 pm

    Well I took it to the repair shop and he fixed the gas leak and he said the fuel pressure was under 20 and it suppose to be higher..then he said I might need a fuel pump but when me and another mechanic checked everything out he said its a vaccum leak..so im confused as far as what to do or who to believe.

  29. dennisb - Auto Tool Sales :

    Date: October 18, 2011 @ 6:15 pm

    Kita, I would find a reputable shop and stick with them. They should be able to make the repairs and check the emissions readings in the shop so that you know you’ll pass before going back for inspection. If they don’t have the equipment or the confidence – then find a shop that does. Good Luck.

  30. Kered :

    Date: October 24, 2011 @ 8:35 pm

    Ford Expedition with po171 code.
    Replaced O2 sensor bank 2
    Replaced MAF
    Replaced PCV
    Replaced PCV elbow ( found leak)
    Replaced spark plugs

    Car runs rough under 40 mph on hwy.
    Runs great with 1/2 tank full on hwy but still feeling stuttering and misfire. I apply gas pedal which corrects problem temporarily.
    Any Suggestions ????

  31. dennisb - Auto Tool Sales :

    Date: October 25, 2011 @ 6:48 am

    Kered, since you say it runs great with 1/2 tank full, you may have a venting issue (not sure if I’m misinterpreting what you are trying to say) . Otherwise if I were checking it I would smoke test for more vacuum leaks and check Mode 6 for misfires that may not have set a misfire code yet. Good Luck.

  32. Miranda :

    Date: November 21, 2011 @ 6:14 pm

    I feel like a fool, so listen up people!! I have an 04 Mazda Tribute…sure enough I was also driving around town when my engine light came on…hmm 2 days later my car started rough idling at a stop in park or in drive. I looked up the P0171 and p0174, and read the research about vaccum leaks too. I checked for vacuum leaks and found none, moved on to cleaning the MAF sensor and carb, air filter, spark plugs and wires (which were fine..just in my frustration i thought it couldnt hurt), I was SOO close to, dropping $100 bucks for a new oxygen sensor…if that didnt work, I was going to give up and put it in a shop…but sure enough it WAS a vacuum leak!, right on top toward the back there was an ‘elbow’ hose pinched together why i never heard or saw this I just don’t know!!! but check people check with your eyes and ears before spending money like me! BTW codes are clear she’s running beautifully! Good luck all! p.s mine sounded like somebody letting the air out of a tire real high pitched, maybe I couldnt hear it until it got worse i just dont know

  33. oscar ortez :

    Date: November 26, 2011 @ 8:53 pm

    I had a code p0171 and p0174 on my 2001 ford expedition. and same thing It was a vacuum leak on the back of the intake. Thanks to you guys. If everybody would share a litle bit of our knowledge we will have a better world…

  34. Mike :

    Date: January 30, 2012 @ 11:14 am

    2003 5.7 Silverado. For months now, 171 and 174. I have replaced plugs, wires, MAF, fuel filter, evac canister (by shop). They indicated there was no leak. Still runs rough and stalls when cold. Codes will not go away. Any other ideas before I replace the CAT Converter?

  35. dennisb - Auto Tool Sales :

    Date: January 30, 2012 @ 11:27 am

    Mike, some vacuum leaks are hard to find. Some only leak when the engine is cold which would make since because your’s runs rough when cold. When the engine heats up the expansion could make the vacuum leak very hard to find. I recommend taking to a shop that has a smoke machine and allow them to keep the vehicle overnight and smoke test it when it’s cold the next morning. Could be intake gaskets, that’s what I would pay special attention to. BTW, lean codes NEVER mean a bad catalytic converter. Good Luck.

  36. RaynsMommy :

    Date: February 26, 2012 @ 1:29 am

    Hi Dennisb,
    First I have to say thanks to all the comments on here I feel more prepared to tackle my issue, or should I say the repair shop. So, I would REALLY appreciate your advice because after doing some research, it may very well be that I have a faulty elbow, leaky hose, or dirty filter. I would like to try the inexpensive repairs FIRST, instead of the other way around, especially since the shop suggested the items listed below. Okay, here it goes, I have a 2000 Jaguar S-Type V6 and just a little over a week my check engine light came on. Since then, I noticed my car starting to idle “oddly” while in “Park” or foot on the brake at a stop. It runs fine at all speeds, but, recently over the week it stalled 3 separate times while I in “park” only. It turns right back on with no problem, but of course I want to fix the problem. I have been doing research and I just found this awesome blog and learned that maybe it is a simple repair (crossing fingers and toes) since my car has been “hissing” when the car is cold and in cold weather only. Well, I took it in to the shop earlier today and they said that P0171, P0174, and P0300 registered ( I previously learned what these were when I researched my cars symptoms) and they said that they were trying to isolate which one of the engine sensors was the cause/problem. After a while they came back and asked me if my car had stalled, when, and how often. I told them that only a few times and only while in “park.” 30 minutes later they “recommended” the following under each category. Please note that I didn’t tell them about the “hissing” because I just learned about the potential of it being a “Large Vacuum Leak” So, I would like your advice as to how I should proceed before I give up an arm, a leg and a half!

    GASKETS-ENGINE
    Intake Manifold Gaskets(Both Upper & Lower) *Parts and Labor $591.04
    ENGINE SENSORS
    Remove and Replace PCV Valve
    *Parts and Labor $278.29
    TOTAL with Taxes $893.42

    Needles to say I almost passed out, I say almost because I know its a Jag that typically comes with higher repair costs. Oh yeah, and they told me that I should not drive my car too much because it is misfiring…

    I THANK YOU in advance!

  37. dennisb - Auto Tool Sales :

    Date: February 26, 2012 @ 9:15 am

    RaynsMommy, If I were checking the car I would try to pinpoint the vacuum leak. I remember working on a few Jaguar S Types that had a common issue with the PCV vacuum line assembly leaking vacuum. See the image for the approximate location to look. Running more roughly with the brake on at idle goes along consistently with a vacuum leak since the power brake booster draws more vacuum from the engine to provide assisted/power brake action. When the car is at higher RPM’s this is much less noticeable. *Note; A bad/leaking power brake booster could cause the issue too, but is a much less common failure.

    The PCV (Positive Crankcase Ventilation) valve is not really a sensor like in the estimate you were provided. If the valve is available by itself and not just as an “assembly” with the hose or molded line altogether, they are rather inexpensive. However, I suspect that the PCV for your car is only sold as the more expensive assembly (probably a dealer only part). While it is possible; the PCV valve “itself” is probably not the problem, most likely it’s the line cracked and leaking somewhere. On some Jaguars, the intake must be removed to access and replace the PCV line assem. That is most likely why the repair shop has given you an estimate for the intake seals as well. If intake seals are re-used there’s a chance they would leak ruining the job. So the only professional way to price the job out is to replace, and not take a chance of the intake seals.

    Things you can do.

    1. Check for an obvious vacuum leak in the area I pointed out in the picture. If you find a vacuum line try repairing just the section that is leaking. If a plastic line leaks, cut it and use a rubber hose that snugly makes the union. If a rubber vacuum hose leaks, use a plastic line in the same manner.

    2. If you end up paying the shop to do the repair, make sure you are not paying for duplicate labor. If you are paying for the intake R&R to access the PCV line assembly, you should not have to pay full price (or full boat) for the PCV line labor as well. This is known as double dipping! It’s OK to pay some labor for the PCV line, you just shouldn’t have to pay double. Good Luck.

  38. Sammy :

    Date: March 20, 2012 @ 10:49 am

    I have a 2000 ford focus. zts model. It has a ztec engine. single overhead cam. I have a check engine light on. The code came back as a po171 code. A mechanic sprayed brake cleaner all over the ngine and nothing happened as far as leaks go. He stated if it was a vaccum leak it would make hissing and spraying. My next step is the o2 sensor. Do you think this is the problem? I dont want to spend all kinds of unnesscasary money! Please advise…

    Thanks,

    Sammy

  39. dennisb - Auto Tool Sales :

    Date: March 20, 2012 @ 11:02 am

    Sammy, a lean code is not likely a bad O2 sensor. Like mentioned in the article, an idle change may not be noticed when checking with a spray or propane.

    “Cars that have computers can quickly compensate for the change in fuel mixture. This makes checking for vacuum leaks in this manner more difficult. If you have access to a scan tool, checking the oxygen sensor readings or fuel trim can be done while checking for leaks with propane. If the readings increase when you are adding propane to be suspected leak area, this indicates a vacuum leak.”

    Good Luck

  40. Jerry :

    Date: March 24, 2012 @ 12:34 pm

    No one has mentioned using a vacuum gauge. Can you have a vacuum leak and the gauge reads normal?
    Thanks

  41. dennisb - Auto Tool Sales :

    Date: March 24, 2012 @ 12:58 pm

    Jerry, a vacuum reading that is lower than 17-21″ HG may indicate vacuum leak, however that in itself doesn’t help to locate the leak. If you have both lean codes we already know that there’s a vacuum leak.

  42. Jerry :

    Date: March 24, 2012 @ 1:10 pm

    I was just wondering if that would confirm there is one, or could there be a small leak enough to set lean both banks and still read normal. I like your info very comprehensive and informative. I got pulled over for an outdated inspection sticker and I’m scrambling to fix this to get it inspected. I don’t want to just take it somewhere were they keep throwing parts at it at my expense, I can do that. I’ve cleaned the maf sensor and changed the fuel filter. going to check the fuel pressure next.
    hopefully its not the pump.
    Thanks for your help

  43. dennisb - Auto Tool Sales :

    Date: March 24, 2012 @ 3:45 pm

    Jerry, in my experience low fuel pressure usually is not the cause of these codes. I would look for a vacuum leak when both codes are present. Good Luck.

  44. Jerry :

    Date: March 24, 2012 @ 5:21 pm

    Thanks Dennis, maybe I’ll have the guy do the smoke test again. The first time he didn’t block the intake up front. I’ll let you know what it ends up being if we find it.

  45. Matt :

    Date: April 4, 2012 @ 10:11 am

    I have 2000 ford explorer limited 5.0 that also shows lean codes p0171 and p0174. I changed the fuel filter, plugs and wires, o2 sensors, egr valve, maf sensor and a whole new exhaust. Still runs rough at idle and hesitates while driving. I finally gave up and brought it to a mechanic who did the smoke test and could not find a vaccum leak. He believes its the fuel pump. He did a pressure test and said it was low(30 psi) and is recommending i replace the pump. Im gonna have him do it but if for some reason thats not it what else could it possibly be?? Any help would be greatly appreciated.

  46. dennisb - Auto Tool Sales :

    Date: April 4, 2012 @ 11:51 am

    Matt, although it’s possible, a bad fuel pump rarely causes the lean codes. I would check the air intake hose, PCV line including the 90 degree elbows. The rubber elbows, sometimes will collapse. A smoke test is usually a good way to test for vacuum leaks, but sometimes leaks only show up when the motor is cold. For that reason you may want it re-checked after the vehicle has sat in the shop overnight. Good Luck.

  47. Jerry :

    Date: April 15, 2012 @ 2:36 pm

    Dennis, do you know what the fuel pressure is supposed to be on a 2000 explorer sport 4.0 I’ve been looking on the net and getting different results. I tested mine on the rail and get 39. also should it vary at different rpm’s
    Thanks, Jerry

  48. dennisb - Auto Tool Sales :

    Date: April 15, 2012 @ 4:16 pm

    Jerry, could be a couple of different types of FI systems. I would look in ALL-DATA for the specific year and engine. Short on-line subscriptions are available for just one vehicle (great for do it your-selfers).

    You can access fuel pressure specs along with other repair information. Just click on the Auto Repair Manuals link at the top of this page to find out more about this. Good Luck.

  49. Pat :

    Date: May 17, 2012 @ 2:08 am

    2003 Taurus ses just suddenly stalled in morning just cranks. Got it running but barely took it to shop they found hole in hose. reset computer off I went $200. Now its stalling again and won’t stay running after it warms up. and cranks then finally starts and have to keep gas on. It stalls at every light and frm going in reverse stopping stalls it. Shop says no more vacuum leaks thoroughly checked. MAF sensor new, IAC checked, connections checked, New o2s, new fuel filter,Cleared codes again still having same issue. Very stumpped. Is it a ford ghost issue?

  50. dennisb - Auto Tool Sales :

    Date: May 17, 2012 @ 7:16 am

    Pat, I’m curious to what the shop says might be the problem since they are physically there and can check the car with a scan tool, manual test equipment etc.

    I would check for pending codes, fuel pressure and also make sure the fuel itself is not contaminated. Depending upon where the vacuum leak was in the hose, the MAF could have trash in it… is the air filter bad and allowing unfiltered air to get into the MAF? All these are things to check, but the technician should have some ideas of his own (if he doesn’t find one that does). Good Luck.

  51. Pat :

    Date: May 22, 2012 @ 1:44 pm

    Checked all items you mentioned all good. Throttle body cleaned prestine. PCV hose is all new. Going to dealer this time guess time to shell out big bucks probably for a pinhole vacuum leak. Also tried the SEA FOAM in the vacuum system but still same issue. Codes still showing lean mix. When they find it ill update you thanks for your post.

  52. Tony Lestan :

    Date: May 29, 2012 @ 7:59 am

    My 1999 4.0 had these same codes and I ran through the usual suspects. It did end up being the fuel pump, although the truck ran fine with 30 PSI, I learned that this year has the regulator in the tank and the single line system needs about twice that pressure. Problem solved for $50 with a pump from a boneyard. Cut through the floor under the back seat to make the swap painless and easy to do in the future.

  53. dennisb - Auto Tool Sales :

    Date: May 30, 2012 @ 2:28 pm

    Tony, thanks for the comment. Cutting through the floor to access the fuel pump is an interesting idea. I would like to point out to our readers that doing this may cause a problem with gas entering the passenger compartment in the event of an accident. Some vehicles are designed with access holes, but they also have sealed covers with enough bolts to secure them properly. This reduces the risk of fuel gushing into the passenger compartment during an accident.

  54. james :

    Date: May 30, 2012 @ 9:36 pm

    I have a 2009 chevy express with the 4.8 I recently had the check engine light come on and had the dealership check the codes they came back lean bank 1 and 2 the van would die out when it came to a stop sign after several hours of driving but would always restart they said dirty throttle body they cleaned it and the van stopped stalling but lean codes still come on and still dosent idle like i think it should not totally smooth also it has a little problem starting in the morning dosent fire crisp kinda spits and sputters but once warmed up no problems 115000 miles.

  55. Pat :

    Date: June 6, 2012 @ 12:49 am

    Regarding 2003 Taurus SES stalling issues – All vacuum lines and hoses replaced. I may have found another source the air box has one clip broken on it possibly letting air not filtered into intake. Also new codes showed up after car ran rough again and stalled many times. P0303 P0316 but injectors are good. So if airbox is still tight my next thougbt could be a mechanical issue not seen by a sensor.

  56. Ben :

    Date: June 14, 2012 @ 2:49 pm

    This is a great site with the best info I’ve found yet. I have a 97 expedition 4.6L I’m getting alternating lean codes (both sides but not at same time) the 4.6 that’s in it is out of a 95 T-bird as fords great planned obsolescence rendered the original 4.6 undrivable right on queue at 160k miles … ok anyway I’m also getting the exhaust gas recirculation low code believing this is the sensor with 2 hoses thingy (not up on terminology) and possibly fuel pressure regulator. As having done a fuel pressure test following the Haynes repair manual I do know the regulator is not at optimum pressure but pump is fine. Wondering if one thing could cause both issues or if both items or am I aiming way off? (have parts from the 95 and 97 to try b4 buying parts) Thank you.

  57. dennisb - Auto Tool Sales :

    Date: June 14, 2012 @ 3:10 pm

    The lean codes and EGR low flow are not related. One really has nothing to do with the other. Low fuel pressure can sometimes in rare cases cause lean codes. See our other articles on lean codes. The DPFE could cause the low flow codes, however plugged EGR passages in the intake can too. Good Luck.

  58. Ben :

    Date: June 14, 2012 @ 3:30 pm

    Is it possible to test the DPFE with a multi meter? Where I do have the one off the 95 and one test would be put it on and see if it fixes it really if it doesn’t I cant be sure its the problem as you said could be a couple other things. Having read the EGR article my thinking on the regulator is its easier access then all the vacume lines and with the computer the vacuum lines will be a pain to test the egr passages seemed clear having blew through all passages with a compressor before reinstalling the intake and examining vacuum hoses and lines during reassembly. I’m mainly confused bout both these parts being original 97 and not having this problem before the timing chain went out.

  59. dennisb - Auto Tool Sales :

    Date: June 14, 2012 @ 3:49 pm

    Ben, there’s no multi-meter test. The DPFE typically corrodes inside and fails. 15 year old DPFE’s are very uncommon. Most get changed out after a couple of years.

    Simply blowing through lines with compressor is no indication of how clear the EGR passages are. They can be partially blocked and set the trouble code. Good Luck.

  60. Ben :

    Date: June 14, 2012 @ 3:52 pm

    OK thank you. Will see what I can do tomorrow after work. I appreciate all the info thank you much.

  61. keith :

    Date: July 9, 2012 @ 1:11 am

    Hi, I have codes p0171 and p0174. I have changed pcv,fuel filter, and cleaned the MAF. I tested the TPS and found it to be backwards. On closed I read 4.0 vdc and on open its reads 0.5 vdc. It should be the opposite. I also noticed that when the air filter was off and I ran the engine it idled very rough, but putting the air filter back on it idled a lot smoother. It does have a little hesitation every so often on idle.

  62. dennisb - Auto Tool Sales :

    Date: July 9, 2012 @ 6:30 am

    Keith, running the vehicle without an air filter could have damaged the MAF (Mass Air Flow) sensor. I would also check for vacuum leaks. Since you have no TPS failure code (the computer sees no error or fault) I would not worry about your manual readings that seem to be backwards. Good Luck.

  63. keith :

    Date: July 12, 2012 @ 10:19 pm

    Thanks for your help. Man I found a hole bunch of vacuum leaks. I did a smoke test. Thanks again.

  64. Reagan :

    Date: July 15, 2012 @ 2:35 pm

    I have a 2003 Jeep Cherokee 4.7 4wd.
    Just got it for the wife to run the wheels off of.
    I noticed a hissing sound from the rear when I got it. Decided to replace the fuel filter/booster and it still hissed. Only drove it maybe 200 miles over a week and suddenly did not want to start easily and threw the 0174 codes and the other showing that both banks were running lean. idles rough until running for a few seconds and revving it. Then does not want to accelerate for several seconds. Then seems to drive normal until you try to punch the gas. Found a leak on the vacuum hose near the fuel filter cut off the broken part and reattached. no hiss, but still doesnt want to start easily and acts the same. I’m guessing the lean problem combined with me punching it caused po301 and po302 misfire codes. Cleaned the throttle body as well.
    Any ideas?

  65. dennisb - Auto Tool Sales :

    Date: July 15, 2012 @ 4:26 pm

    Reagan, the 4.7 does not have a common issue with vacuum leaks. I would check the fuel pressure and check to make sure the catalytic converters aren’t restricting the exhaust. You said “seems to drive normal until you try to punch the gas” that makes me think clogged exhaust. That’s a common descrpition of the problem, when the exhaust cannot allow enough flow. If restricted badly enough misfire codes can be triggered also. Good Luck.

  66. Sindee :

    Date: July 17, 2012 @ 6:42 pm

    My 2003 Ford Expedition is coming up with codes: P0171, system too Lean Bank 1 & also P0316 Miss Fire Detected. Can you help me understand what this means ? Thnx!!!

  67. dennisb - Auto Tool Sales :

    Date: July 17, 2012 @ 7:37 pm

    Sindee, the article goes over causes for lean conditions. The P0316 means there was misfires shortly after start-up. They could be related. If a vehicle has a bad enough vacuum leak, it could cause a misfire. Someone needs to check the vehicle in person. Good Luck.

  68. Li :

    Date: July 17, 2012 @ 10:17 pm

    My 2010 Ford 150 (STX 4.6L 2valve engine) stalls when I make turns. It happens intermittenly. The throttle body was changed, but it does not solve the proble. The dealer could not duplicate the problem or read the code at the beggining. Last weekend they connected a recorder to the truck and successfully record the codes when the truck stalled again. They are P0171, 174 and 316. Besides, FEP: -0.462psi, BARO: 14.355psi, LFT1: 29.69%, SFT1: 4.69%, SFT2:3.13% and LFT2:26.56%. Can you help me out? Many thanks in advance.

  69. dennisb - Auto Tool Sales :

    Date: July 18, 2012 @ 7:20 am

    Li, the dealer that is working on your vehicle obviously is better equipped to diagnose your problem than I am. However, from the problem you’ve described I would consider the possibility of broken baffles inside the fuel tank. The lean codes usually are caused by vacuum leaks, but could be caused by fuel starvation. There are baffles in most fuel tanks that keep fuel surrounding the the pick up screen for the fuel pump. If those baffles have broken, the fuel will all rush to one side upon turns. The lower the fuel level to begin with, the better chance that the fuel pick-up screen will be left dry during turns. I’ve seen broken baffles before. They are usually plastic and can become brittle and break. Loose pieces inside the fuel tank can damage the fuel pick-up sock/screen. This can ultimately damage the fuel pump itself over time. Good Luck.

  70. Eric G :

    Date: July 21, 2012 @ 1:34 pm

    I have a 1997 Pontiac Sunfire with 134k mileage.Since I bought it about three weeks ago, its check engine has been on. Had it checked at Auto Zone and it kicked out P0171 code. Aside for the check engine light. It sometimes hesitates at stop lights, and burns through fuel pretty fast. My parents live maybe 3 miles away and when I drive to their house the fuel needle literally drops almost half a mark. I plan on checking the vacuum hoses and some other things this weekend, but do you have any thoughts other than that? Thanks in advance, you seem very knowledgeable.

  71. dennisb - Auto Tool Sales :

    Date: July 21, 2012 @ 4:23 pm

    Eric, no common vacuum leaks come to mind on the type of car you have. It wouldn’t hurt to look for obvious vacuum leaks though. I would also make sure the MAF (Mass Air Flow) and air filter are clean. Look for air leaks between the MAF and the throttle body. This could be allowing un-metered air into the engine causing the code. I would also clean the throttle plates for the poor idle problem to see if that helps. There are other potential causes of lean codes, like exhaust leaks, the fuel pump could be going out or the injectors could be getting clogged. A fuel pressure test would be the next logical step if problems persist.

    Gas Mileage

    Watching the needle move while driving doesn’t tell you how many miles per gallon you’re getting. The needle may also bounce a little while driving, further altering any perceived idea about how good or bad the gas mileage is. The gas needle on some cars stay at full for an extended time when first filled up. Some car’s gas gauge needles travel slower towards the top end of the gauge as compared to when the needle goes below the 1/2 way mark. The only accurate way to check the gas mileage on your car is to do it manually. Fill the tank and write the odometer miles down. On the next fill-up, calculate how many miles you’ve traveled and divide by the gallons. Good Luck.

  72. Troy :

    Date: September 4, 2012 @ 5:50 pm

    Have a 96 Ford Taurus…has 1131, 0305, 0306, 0171, 0174 and 0420 codes. The car when hot outside will drive for 20 minutes, check engine light will flash, and the car will sputter and die. Once I coast to side of road, the car will start up right away. I’ve changed the coil, plugs and wires and it still does this. Any ideas???

  73. dennisb - Auto Tool Sales :

    Date: September 4, 2012 @ 6:20 pm

    Troy, flashing means that there’s a severe problem causing horrible emissions that may cause damage to the catalytic converter. I would look at the misfire codes first but sounds like you may have several issues and maybe an already damaged converter or two. Good Luck.

  74. Troy :

    Date: September 4, 2012 @ 6:26 pm

    Dennis..could a vacuum leak or egr cause the misfire codes?? I changed out the coil, plugs and wires hoping that was the reason for the misfire…

  75. Troy :

    Date: September 4, 2012 @ 6:32 pm

    And when it does all this…check engine light flashing and dies…right before this the A/C will blow hot air.

  76. dennisb - Auto Tool Sales :

    Date: September 4, 2012 @ 6:35 pm

    Troy the computer is probably shutting down the compressor to save the engine from overheating further. A very bad vacuum leak could cause lean misfires but not on just a couple of the cylinders – it would affect ALL.

  77. Troy :

    Date: September 4, 2012 @ 6:52 pm

    I haven’t gone to get codes pulled since I changed the coil and stuff. Hoping the misfires won’t be there after changing them. So your leaning possibly to catalytic convertor if the misfire codes are gone??

  78. Troy :

    Date: September 4, 2012 @ 7:05 pm

    And it only does this when the outside temperature is 85 or above. It’s drives just fine in the mornings when it’s cooler out. Thank you for all your help Dennis…..

  79. dennisb - Auto Tool Sales :

    Date: September 5, 2012 @ 6:28 am

    Troy, if the vehicle still runs the same I would suspect the codes will still be there or come back quickly after clearing them. I would do a compression test to check the engine’s integrity. You may also have restricted catalytic converters causing part of the problems. Good Luck.

  80. Paula :

    Date: September 16, 2012 @ 5:09 pm

    We have a 1999 Expedition that codes P0171 & P0174 where found at Auto Zone. One of our friends suggested checking the fuel cap. If it wasn’t tight it would cause the Service Engine Light Soon to come on. Sure enough…tightened cap and light went off after about an hour of driving time. Simple things sometime!

  81. dennisb - Auto Tool Sales :

    Date: September 16, 2012 @ 6:34 pm

    Paula, the gas cap loose can cause a code but not for the lean codes. The code for the gas cap loose would be something like “P0455″ meaning a large evaporative system leak. I would check the things in the article when the warning comes back on. Good Luck.

  82. wb :

    Date: October 3, 2012 @ 11:04 pm

    First of all you guys are great. My moms jeep cherokee 2001 running rough at idle. I scanned it, a P0171 lean code. I’m pondering to myself as she is describing what is going on. Something just doesn’t make sense. So I follow vacuum lines. Followed a line to the front bumper. The vacuum canister that was at some point attached to the inside of the bumper, is gone. hahaha. I dunno. She delivers mail and who knows what they do to their vehicles. But anyway, easiest/stupidest vacuum fix ever. (where is the vac canister?? I thought to myself… follow the lines) its gone! Enjoy the chuckle.

  83. dennisb - Auto Tool Sales :

    Date: October 4, 2012 @ 6:50 am

    wb, one could say it just disappeared… it vanished into a vacuum. I know, I won’t give up my day job.

  84. Tom :

    Date: October 13, 2012 @ 10:35 am

    I have a 2003 dodge caravan. The code i am getting is P0171. The symptoms are: the car cranks for about two minutes then finally starts. It runs great once it starts. If i turn the car off and try to restart right away, it will start. If i wait a few minutes then try and restart it then takes a minute or so of cranking before it turns over. Can you help me?

    thanks, tom

  85. dennisb - Auto Tool Sales :

    Date: October 13, 2012 @ 6:49 pm

    Tom, I would check for vacuum leaks and fuel pump pressure. Good Luck.

  86. Victor :

    Date: October 23, 2012 @ 9:28 pm

    I have a 2002 F-150, 4.2L engine with 123,200+ miles. My service engine light has been on for 1 year, since I had it “repaired”. These are the stored codes: P0171, P0174, & P1151. Every time the engine is “repaired” the codes come back; although I am sure what the previous codes have been. I was told about a week ago that I needed to have the gasket replaced on the intake manifold, change the oil and antifreeze—-over $800. HELP!!!

  87. dennisb - Auto Tool Sales :

    Date: October 24, 2012 @ 7:24 am

    Victor, You have lean codes which usually means un-metered air is entering the engine and the computer cannot enrich the air/fuel mixture enough to compensate for it. Vacuum leaks are by far the #1 cause of this. I would check the PCV hose first. The PCV rubber elbow is a very common cause of vacuum leaks. The Mass Air Flow could be dirty causing skewed readings. And finally the upper and lower intake gaskets could be leaking. The bolts that hold the intake are insulated with rubber grommets; be sure these are replaced if the upper and lower intake gaskets are being done. The intake job is typically done with coolant and an oil change because coolant has to be drained before removing the intake and an oil change helps to remove contaminants after having things apart. See the image below for a Technical Service Bulletin (TSB 05-17-3) related to your vehicle. For full access to the TSB see our auto repair manuals page (click on the image below) for a limited subscription.

  88. Kary :

    Date: October 26, 2012 @ 1:55 pm

    1997 Ford Explorer 6 cyl. issues. A few weeks ago it threw a P0300 code. Mechanic said it was a fluke, found nothing wrong. Last week (within 50 miles of the P0300 code going off) it threw the codes P0171 and P0174. Took it back to the mechanic and he can’t find anything actually wrong without just replacing all kinds of things. He changed the MAF and checked for vacuum leaks. Didn’t make a difference changing the MAF and found no vacuum leak through a chemical test. I just had the fuel pump and filter replaced 1700 miles ago. My mechanic suggested taking it to Ford to have them find the issue. I can’t afford to take it there to find out they are going to charge me 900 dollars to replace a 50 dollar part. What do I check next?

  89. Kary :

    Date: October 26, 2012 @ 2:01 pm

    Sorry forgot to give problem description too. On a cold start (every cold start), I have to give it gas and hold it. It runs really rough. If I just start it without immediately giving it constant gas, it stalls immediately. Once it is warmed up, I can let off the gas and it does run but a little rough and rpm’s fluctuate between 300 and 600. It does okay when accelerating or at a constant speed although I can feel what might be a miss. I can’t tell 100% though because it is so slight. When stopping, it idles rough and the rpm’s fluctuate between about 300 and 500.

  90. dennisb - Auto Tool Sales :

    Date: October 26, 2012 @ 2:27 pm

    Kary, I would do a smoke test when COLD. Have the shop keep the vehicle overnight, so they can test it before it has been ran in the morning. I used to do this all the time for vacuum leaks, coolant leaks etc. It’s always best to check the problem when it occurs, not after it’s acting normally. If there’s a vacuum leak, likely from the intake seals, it may not leak after the engine heats up due to expansion. A vacuum leak will cause the lean codes and could cause random misfires throughout all the cylinders until warmed up. I would also have them check the fuel pressure just in case it’s not pumping up to the proper specification. Good Luck.

  91. Victor L :

    Date: November 9, 2012 @ 12:20 pm

    Hi, I have Honda Civic 1998 1.6L DX. Sometimes once a month code P0171 appears. At idle ST fuel trim is good, but once I start to accelerate trim goes to the limit on a highway above 60 mph. The more I press the gas pedal – the more it compensates for the bad condition. Once I release the gas pedal trim goes back to normal. Gas mileage is down by half. What do you think ?

  92. dennisb - Auto Tool Sales :

    Date: November 9, 2012 @ 12:36 pm

    Victor I would check for tears or leaks in the air intake boot. Sometimes acceleration can shift the engine and cause tears in air boots to widen. Also any loose clamps may similarly cause a problem on acceleration. Besides that I would look at the Oxygen sensors. Hondas don’t usually have issues with vacuum leaks but on higher mileage cars, Oxygen sensors are a common failure. Good Luck.

  93. BrandonG :

    Date: November 10, 2012 @ 9:32 am

    Dennis,
    The check engine light on my wife’s 2001 Ford Windstar came on a few weeks ago. I took it to the parts store for a scan and got the P0171 and P0174 codes. After doing some research online, I tried to start with the simple items first. I removed and cleaned the MAF sensor (didn’t really want to replace it if this wasn’t the issue). This didn’t take care of the problem. I did some more research and found out about the TSB on the intake gaskets and shoulder bolts, valve cover etc. I bought the kit and replaced all of that stuff and the PCV Valve assembly. The check engine light went out and all seemed well until the engine got hot and started acting up again. When the engine gets hot, it starts running rough and you have to continuously step on the pedal and rev it to keep it from dying. I had the fuel pump and fuel filters replaced about 4 months ago. The guy at the parts store mentioned something at the time about the “fuel pressure regulator”. Should I invest in the new MAF sensor or focus on finding a vacuum leak. If you think I should focus on the vacuum leak, where would I go about getting the necessary equipment to do this? Thanks in advance

  94. dennisb - Auto Tool Sales :

    Date: November 10, 2012 @ 6:48 pm

    A very common vacuum leak on the Windstar is from the intake runner seals. See our Ford Windstar vacuum leak article for details. Good Luck.

  95. Demetrius :

    Date: November 15, 2012 @ 9:36 am

    I have both lean codes on my 2003 Ford expedition. It rides very ruff and acceleration is very choppy. It has lost tons of power and struggles to go up hills. When it does the check egine light would start to blink and the a/c would get warm until i let off the gas or press lightly on the gas…whats going on?

  96. dennisb - Auto Tool Sales :

    Date: November 15, 2012 @ 10:25 am

    Demetrius, there may be more than one issue but the symptoms you are describing are likely due to a clogged catalytic converter. Those are classic symptoms. You may ask – How do you check a catalytic converter for being clogged or restricted? Measuring the exhaust back pressure is the answer. Click on the image below for details on one of the testers we have for sale.
    Catalytic Converter back pressure tester.

  97. Orlando :

    Date: November 21, 2012 @ 9:11 pm

    Hello I have a ford explorer 2000 it shows code P0171 and it idles in Parking or Neutral when turning to Reverse or Drive it feels perfect, what could it be?

  98. dennisb - Auto Tool Sales :

    Date: November 22, 2012 @ 8:05 am

    Orlando, I would check for vacuum leaks. On the Explorer the upper intake (seals/gaskets) are a common issue. Good Luck

  99. Orlando :

    Date: November 22, 2012 @ 9:27 am

    Thanks my friend i wll check for some leaks then, i’ll comment later if i found the problem. Have a nice day.

  100. Charmaine :

    Date: November 22, 2012 @ 12:29 pm

    I have a 98 Ford Expedition and I took it to Auto Zone due to check engine light on. The codes PO171 and PO174 were pulled up. I checked the vacuum hose and it has wear to it with cracks…etc. What is the part name that I need to purchase to replace this hose?

  101. dennisb - Auto Tool Sales :

    Date: November 23, 2012 @ 8:11 am

    Charmaine, Possibly the PVC line assembly. Good Luck

  102. Joey :

    Date: November 28, 2012 @ 11:30 am

    I have a 2000 ford explorer, it just failed emissions expection. with the codes, P0171 and P0174. for over a year the check engine light has come on and off, I had a fuel inject cleaner done, and it went off and hasn’t come back on. It idles and pings ALOT when stopped on park, etc. i’m beginning to think after reading your AWESOME site, that it might be a vacuum leak, put sad thing is, I 90% clueless when it comes to cars. HELP me please, and thank you for your help.

  103. dennisb - Auto Tool Sales :

    Date: November 28, 2012 @ 11:40 am

    Joey, the most common vacuum leak on Explorers (around your model year) is from the intake manifold seals. They are small rubber type seals that fit into recessed grooves on the underside of the upper intake. At different temps they may not leak. Meaning it may be best to check for a vacuum leak when the engine is cold and heat expansion has not sealed the gaps temporarily. You may want to leave the vehicle at the shop overnight so the technician can check it cold. Let him know that you would like it checked as cold as possible for this reason. He may use FI (Fuel Injection) spray or a smoke machine. Good Luck.

  104. Orlando :

    Date: November 30, 2012 @ 3:56 pm

    Hey I found failure in my explorer 2000 in fact it was a vacuum leak, I bought some carb-clean and spray it around the upper engine running and it leak, so I took the intake out and put some engine sealer took me like and hour now my car feels great, thank you very much Dennis, you are a great dude.

  105. dennisb - Auto Tool Sales :

    Date: November 30, 2012 @ 5:20 pm

    Orlando, that’s great. Have nice weekend.

  106. Eddie :

    Date: December 5, 2012 @ 9:05 pm

    I have 1999 f150 v8 4.6l showing a P1151 code. I have changed out the sensor with a new one on drivers side closest to the bumper, wiped out the codes with scanner but the code still shows up. Could this be caused a vacuum leak in the pcv elbow? At first I didn’t think so because only one code showed up. What could it be?

  107. dennisb - Auto Tool Sales :

    Date: December 6, 2012 @ 12:35 pm

    Eddie, I would double check to make sure the correct sensor (there’s four total) was replaced. Good Luck.

  108. Dick :

    Date: December 6, 2012 @ 1:57 pm

    Yesterday I read your advice about looking for a vacuum leak on my 02 Ford Ranger w/4.0L. I found a leak in the elbow that sits right under the top radiator hose. New elbow with line cost $36 at Ford dealership. Much cheaper than starting to replace 02 sensors. Thanks for your help!

  109. dennisb - Auto Tool Sales :

    Date: December 6, 2012 @ 2:12 pm

    That’s great. Thanks for letting us know how it worked out.

  110. Orlando :

    Date: December 7, 2012 @ 11:38 am

    Hey Dennis, my car (Explorer 2000) is showing a code and it says P1309 No DTC definition found. See service manual. What does this one mean? Thank you.

  111. dennisb - Auto Tool Sales :

    Date: December 7, 2012 @ 2:43 pm

    Orlando, The P1309 is for the camshaft position sensor. When Ford first made the engine a DIS (Distributor-less Ignition System). They simply used a piece that sat where the distributor used to go. This is now called a cam synchronizer (running off the camshaft). They placed a half moon piece at the top. And a Hall Effect sensor above that to count the revolutions. A common problem is that the 1/2  moon piece becomes distorted. Since there’s not much clearance, it can interfere with cam sensor and ruin it in the process. The computer no longer receives the expected signal, setting off the code. It’s important to install the replacement cam synchronizer in the same clock position, or a code may be triggered again. Good Luck.

  112. Orlando :

    Date: December 9, 2012 @ 10:05 am

    Thank you Dennis, in what part of the engine its the camshaft synchronizer located?

  113. dennisb - Auto Tool Sales :

    Date: December 9, 2012 @ 10:14 am

    Orlando, if it’s like the one in the diagram it’s in the rear. Check a repair manual if you don’t see it. Good Luck.

  114. Jerry :

    Date: December 13, 2012 @ 4:16 pm

    hi dennis,
    my cel is on for p0171 and p0174. i have a 2001 f150 4.2L v6 supercrew truck with only 122k miles. i had a side mechanic change the upper and lower intake gaskets and fix a coolant leak. then my light went off. about 15 miles later, suprise, its back on for the same thing! what the mechanic did really helped my truck tho. it took the rough idle out of it. so i had the spark plugs and oil changed at a shop. i then started looking for possible causes. i changed the air filter cause it was dirty, i put lucas gas treatment in the gas tank and i cleaned the maf sensor with cleaner. i reset the check engine light after i did the mass air flow sensor. it went probably around 15 miles. then on the way home, it came back on again. my side mechanic tried to find a vaccuum leak when he worked on the manifold gaskets but couldnt find one. i really dont know where to go from here. and i dont want to put a lot more money into it. thank you.

  115. dennisb - Auto Tool Sales :

    Date: December 13, 2012 @ 5:05 pm

    Jerry, After clearing the codes, if the computer sees the same fault is present it will set the code. I would recommend having a shop smoke test it and find the remaining vacuum leak. Good Luck.

  116. Jerry :

    Date: December 13, 2012 @ 5:41 pm

    have any ideas what else it could be?

  117. Jerry :

    Date: December 13, 2012 @ 5:42 pm

    you dont think it would be the o2 sensors?

  118. Jerry :

    Date: December 13, 2012 @ 5:44 pm

    ive read about 4 elbows on the pvc line also.

  119. dennisb - Auto Tool Sales :

    Date: December 13, 2012 @ 6:52 pm

    Like I said in the article usually it’s not the Oxygen sensors. It’s usually a vacuum leak. A smoke test if properly done will help find the leak, no matter if it’s a PCV elbow or anything else.

  120. Todd :

    Date: January 1, 2013 @ 4:06 pm

    I have a 2003 silverado ss 6.0,codes p0171 and p0174 come on and go off. I have replaced the mass airflow sensor,and the air filter,no leaks found. I checked the intake manifold for leaks with no luck. While doing this check i broke one of the intake bolts, please help.

  121. Todd :

    Date: January 1, 2013 @ 4:09 pm

    The bolt has some type of sleeve on it.

  122. dennisb - Auto Tool Sales :

    Date: January 2, 2013 @ 7:08 am

    Todd, most likely the intake gaskets/seals. A smoke test if properly done will help find the leak. You may need to have the intake removed to replace the bolt and sleeve. The dealer will most likely have to order the bolt because this is not a “wear” item and usually never has to be replaced. Good Luck.

  123. Taylor :

    Date: January 10, 2013 @ 10:38 am

    I have a ford explorer sport and the check engine light has been on with the codes p0171 and p0174.It runs fine at higher speeds but sometimes it idles rough when I’m sitting at a stoplight.It shut off on me once but started right back up.What could be going wrong?

  124. dennisb - Auto Tool Sales :

    Date: January 10, 2013 @ 10:54 am

    “the most likely cause is a vacuum leak”

  125. Taylor :

    Date: January 10, 2013 @ 10:59 am

    If I notice that a hose is a little cracked but not all the way through should I still replace it?

  126. dennisb - Auto Tool Sales :

    Date: January 10, 2013 @ 12:27 pm

    Taylor, I would replace it. Sometimes they can cause problems (like a restriction) even if the are not leaking yet. I would also do a smoke test to see where the vacuum leak is. Good Luck.

  127. Orlando :

    Date: February 2, 2013 @ 12:52 pm

    Hello Dennis I changed the camshaft position sensor and synchronizer my car is doing OK now (Explorer 2000 V6 4.0) But the check engine light came back on showing P1309 ‘Misfire Detection Monitor not enabled’ what do I have to do to fix this?

  128. dennisb - Auto Tool Sales :

    Date: February 2, 2013 @ 5:28 pm

    Orlando, most likely the synchronizer was not installed at the proper clock position. I would refer to a manual on how to install it properly. Good Luck.

  129. Dave S. :

    Date: February 4, 2013 @ 4:41 pm

    Had a nightmare 2000 mile trip getting 7 MPG in a 1999 Jeep Grand Cherokee. P0171 code was showing up with my code reader. Just for maintenance purposes I replaced spark plugs before trip. Half way on trip I had a mechanic work on it and told him about the P0171 code and a P0108 I had seen once. He replace #1 injector, pulled and checked plugs I put in 2 days before. Got 5 miles down the road and it was again stumbling bad between 45 to 70 MPH. Worst at an even speed – starts bucking so bad you think you’ll break the U-joint. Lots of black smoke on acceleration. Poor idle and terrible acceleration although if you put the pedal to the metal it seems smooth (along with lots of smoke). Plugs covered in black soot but not oily at all (apparent excess fuel). I didn’t have access to other info at the time and when it pulled a P0301 code I decided to swap the ignition coil pack with no luck. That code has not shown up again but it still runs the same. When I got home I put on a vacuum gauge. About 14 PSI at idle of around 600 rpm. Drops to around 2 PSI when I punch the accelerator. Highest it goes after letting off accel is about 21-22 PSI. This problem developed slowly after starting trip but had poor idle before that and also an occasional stumble on slight inclines.

  130. Dave S. :

    Date: February 4, 2013 @ 4:46 pm

    BTW it’s an inline 6 cylinder 4.0 so I think that’s why it only gets the P0171 and no P0174.

  131. Dave S. :

    Date: February 4, 2013 @ 4:55 pm

    I forgot one other thing the mechanic did replace the fuel filter also. Thanks for any help with this.

  132. dennisb - Auto Tool Sales :

    Date: February 4, 2013 @ 5:39 pm

    Dave, the codes may not be a direct indication of the problem with your Jeep. The P0108 is a MAP sensor code. The P0171 is a lean code. There are several people on Jeep forums that have replaced the MAP sensor and ECM (Electronic Control Unit) for this particular problem. The MAP sensor and ECU hardly ever fail on the Jeep Grand Cherokee. A common failure is the crank position sensor and it’s not that expensive. Not saying that it will fix the problem for sure. But I would replace the CKP (Crank Position Sensor) and go from there. I know it is a common problem that does not always set a code. If this doesn’t fix the issue it may be time to let the Jeep dealership diagnose it rather than throwing expensive parts at it. Good Luck.

  133. Dave S. :

    Date: February 4, 2013 @ 6:38 pm

    Thanks very much for the quick response. I considered a CKP but there was no code indicating that so I skipped over that. Good to know they don’t always throw a code. I’ll probably try that next. I forgot to mention I did replace the MAP sensor but it didn’t change anything and fortunately I was able to return it as the parts dealer was okay with returning if it didn’t fix the problem. Hoping it’s not the ECU ;) Thanks again.

  134. Dave S. :

    Date: February 4, 2013 @ 8:23 pm

    I just realized I was thinking of the camshaft position sensor which is easy to replace. My Jeep has both a camshaft position sensor and a crankshaft position sensor. The crankshaft sensor looks very difficult to replace. Do you think the replacing the camshaft sensor would help or does it need the be the crankshaft sensor?

  135. dennisb - Auto Tool Sales :

    Date: February 4, 2013 @ 8:59 pm

    CKP.

  136. Richard :

    Date: February 13, 2013 @ 3:37 am

    ’96 Ford F-250 7.5L w/ check engine light on. Showing PO171 & PO174 according to dealer; they recommend replacing MAF & air filter. I pulled MAF and cleaned w/proper spray cleaner. Also tested MAF function with multimeter as per easyautodiagnostics.com The tests all passed including the signal test which rose and fell with rpm’s linearly without spikes. Pulled the air filter and it was filthy…

  137. dennisb - Auto Tool Sales :

    Date: February 13, 2013 @ 6:33 am

    Richard, if the codes return there’s most likely a vacuum leak. Good Luck.

  138. Rikeyna :

    Date: February 26, 2013 @ 9:38 pm

    Hi Dennis, I own a 2003 Jaguar X-type, I recently took my vehicle to the shop because the check engine and cruise control light came on…It read these codes: P0171, P0174, P0102, P0112, P0455, and P1000. The shop replaced the water pump, coolant/antifreeze, oil filter, oil change, booster vacuum hose, repaired evap hose. Two days of driving the car the engine system fault tranmission fault light came on and the begin to idle while in park or stopped at the light. I took it to the shop for a smog after driving the car for two weeks and two of the computers hadn’t reset. Can you tell me what is causing the car to do this please? Now the check engine light is back on?

  139. dennisb - Auto Tool Sales :

    Date: February 27, 2013 @ 7:08 am

    First off, the water pump and oil change had nothing to do with the codes. I would scan to see what codes are present now. If the P0102 which means “airflow volume to low” and the P0112 which means “intake temperature value to low” are still on accompanying the stalling issue mentioned, it’s possible that the MAF (Mass Air Flow) meter is dirty or going bad. I would also make sure the air filter and throttle plates are clean when/if replacing the MAF.

    If all the codes are now gone except the P1000 there’s probably not a fault to worry about. The P1000 will not cause the CEL (Check Engine Light) to come on. This code is fairly common and does not mean there’s an issue. It has to do with the computer running a self check which can take several drive cycles meeting specific criteria. The vehicle has to be driven certain lengths of time and under certain conditions before the computer will finally remove the P1000. Good Luck.

  140. Byron R. :

    Date: February 27, 2013 @ 11:13 am

    Hi Dennis, I have a 2004 Expedition 4.6L that started to get sluggish bad gas mileage then cel codes P0171 P0174, Don’t laugh tried the smoke test with a Cigar then a small air compressor for a air mattress didn’t find a leak, cleaned the MAF took car battery off, started and ran the car codes came back, changed MAF and fuel filter, Ran the car again and the codes came back Again. Any help?

  141. dennisb - Auto Tool Sales :

    Date: February 27, 2013 @ 11:25 am

    Byron, stay away from the funny cigars. The codes indicate a lean condition most likely from a vacuum leak. The leak could be only when cold. I suggest letting a shop keep the vehicle over-night so it can be properly smoke checked in the morning when it is cool. BTW smoke machines only put in about 1 psi, anything more may create more leaks. Good Luck.

  142. Byron R. :

    Date: February 28, 2013 @ 8:48 am

    Hi Dennis,I found a rubber hose elbow that collapse dimple inward on the back of intake the connects via rubber and plastic hose to the driver side valve cover, took battery off for 15 mins, turn car on No CEL after 2 hours of driving.

  143. Harrison :

    Date: March 4, 2013 @ 1:07 am

    I have a 1993 Toyota Corolla which failed smog and has a code 25, lean exhaust. My mechanic friend just replaced the exhaust manifold (original had cracked), O2 sensor, fuel filter, plugs and wires and distributor cap + rotor and after replacing the parts it didn’t have a check engine light for about 40 miles of driving, but the light is back and it’s the same code. We’re stumped and I figured I’d try asking here what the best course of action from here would be. Thanks for your time.

  144. dennisb - Auto Tool Sales :

    Date: March 4, 2013 @ 7:48 am

    On vehicles built before 1996, 2 digit fault codes are set by the OBD I system instead of more comprehensive 4 digit P (power train) codes on vehicles built 1996 and later that have OBD II systems. The two digit trouble code 25 on a Toyota, means that the air-fuel ratio is lean (too much Oxygen/Air detected in the exhaust). The fault can many times be a bad O2 (Oxygen Sensor) sensor, also called an Air Fuel Sensor on Toyota. If the sensor is good and is detecting too much Oxygen/Air in the exhaust, it could be from a vacuum leak or fresh air leak past the MAF (Mass Air Flow) sensor on vehicles that are equipped with them. If any other codes are present I would also consider those. BTW – the reason the code was not triggered for awhile after it was cleared is because the computer requires some time, called drive cycles in order to monitor data and determine if there is a fault present after codes have been cleared. If the data monitored is not within a certain predetermined range, called parameters the code will be set and the CEL (Check Engine Light) will come on. Good Luck

  145. tony :

    Date: March 9, 2013 @ 7:53 am

    Hey Dennis I have a 02 Camaro SS my engine light came on I scanned it and the codes were oxygen sensor 1 & 2 … I hear a hissing sound but didn’t no if it’s from timing belts. . what are the prices?

  146. dennisb - Auto Tool Sales :

    Date: March 9, 2013 @ 9:18 am

    Tony, if you have P0171 and P0174 the sensors may be indicating too much air/Oxygen in the exhaust. This could mean a vacuum leak. As far as timing belts; they would NOT cause lean codes. Besides Camaro’s are not equipped with timing belts.

  147. tony :

    Date: March 11, 2013 @ 8:57 am

    Thanks.. and I apologize I am new to this stuff. Whats the price range if it happens to be a vacuum leak?

  148. dennisb - Auto Tool Sales :

    Date: March 11, 2013 @ 10:29 am

    Tony, it all depends on what is leaking. A vacuum hose would be a few dollars. An intake gasket several hundred dollars including labor. Good Luck.

  149. lisa :

    Date: March 18, 2013 @ 5:04 pm

    I have a 2003 Ford Windstar. The check engine light came on several months ago and I had it checked and it read lean gas. It idles rough, and when starting to drive from a stop position sometimes it spits and sputters. When I get it to 30 -43 mph speed it feels that the engine is shaking so I let off the gas pedal and it stops shaking, when I give it more gas it’s starts back to shaking usually at 40 mph.. Now I’m having problems with the gas pedal feeling a little stiff when pressing the gas pedal. Sometimes it has power when going up a hill and sometimes not. We have replaced the fuel pump with a used one, fuel filter with a new one and cleaned the fuel injectors. Checked the pressure on fuel and it is 35psi at idle. It is also using oil. Please help I am at my wit’s end!!!!!

  150. dennisb - Auto Tool Sales :

    Date: March 18, 2013 @ 6:40 pm

    I would check specifications for fuel pump pressure – I think it should be more like 45 PSI. Also a very common vacuum leak on the Windstar is from the intake runner seals. See our Ford Windstar vacuum leak article for details. Low fuel pressure and vacuum leaks can cause the lean codes. Good Luck.

  151. Orlando :

    Date: March 21, 2013 @ 3:15 pm

    Hey Dennis, I changed the Cam synchronizer and put it in the right position, have no problems with that now, the engine light has not shown up. The problem now is that the car feels little rough on Drive with RPM’s between 800-1200, I have changed spark plugs, wires, filters, IAC, all seal and gaskets, but it stills feel rough… I don’t know what else to do? As always thank you for your help.

  152. dennisb - Auto Tool Sales :

    Date: March 22, 2013 @ 7:37 am

    Orlando, as long as there are no trouble codes set after sufficient time has passed since the last repair; which insures that the drive cycles have been completed and the computer has not detected any faults. There may not be any tune up or drive-ability parts to change. Some vehicles will normally have a little vibration. After doing so much work on the vehicle sometimes we are quicker to notice every little sensation. However, if the vibration unbearable, there could be other things that can cause vibrations as well. Click the following to see our article on Engine Vibration Causes.

  153. david ray :

    Date: March 31, 2013 @ 4:12 pm

    If your explorer is hard to start when cold for the first minute or so, idles rough, and sets codes po171 and or po174 I strongly recommend getting your timing chain checked immediatly! Mine did this for a month, I didnt get it checked, timing chain broke and stranded me now I need a new motor! Apparently the timing chains were bad from 1998 to 2002 some broke as early as 60,000 miles. Just trying to save others a big bill. lol thanks

  154. dennisb - Auto Tool Sales :

    Date: April 1, 2013 @ 6:31 am

    David, thanks for the comment. The timing chain guides were a problem in those years. The plastic guide towards the top on the cylinder on the drivers side would break prematurely. I never had one set a code though. And it wouldn’t make sense for there to be a lean code set for a possible timing issue. Perhaps there were two problems that was going on at the same time. Thanks again for leaving the comment.

  155. Justin :

    Date: April 7, 2013 @ 7:47 pm

    Hey Dennis, I’ve read a lot of your responses and you should be in the hall of fame for honest auto troubleshooting answers. OK that said, Here goes. I have a 2003 chevy P/u 4.8 V8.230,000 miles, well maintained. It sat in storage for 2 mons. It had and still does have an acceleration prob intermittently.pops n stammers 1-2 g rpm n up. I changed the fuel filter, new fuel n heet. still probs. Afraid to drive far. It was run low on fuel afew times. Hate to drop tank, looked for vacuum leaks. Any advice? Thankx

  156. dennisb - Auto Tool Sales :

    Date: April 8, 2013 @ 11:04 am

    Thanks Justin. I would suspect a fuel pump starting to go out. You could do a fuel pressure test when the problem is occurring to verify this. And yes dropping the tank is no fun, especially when you are working on the floor in your driveway or home garage. Even when I worked in a fully equipped shop with lifts and transmission jacks available, many times I would NOT drop the tank. It is much easier and less messy to either remove the bed entirely (if you have several guys to help) or if working alone, tilt the bed.

    The image below shows the bed removed more than you really need to. Just tilt the bed sideways or towards the rear being careful not to scratch the paint or stretch wires that go the the tail lights. Make sure the bed is securely propped so you don’t end up with a bump on your head. Take note when putting the tank back that the gap is correct between the cab and the bed. Good Luck.

  157. DanC :

    Date: April 9, 2013 @ 6:37 am

    Hi, having issues with my 02 F150, 4.2. Throwing codes P0171, P0174, P0316, P0303. Two weeks ago, I replaced plugs, wires, o2 sensors (all Motorcraft) as well as upper intake gaskets, pcv hose and grommet. Ran like a champ until yesterday, started “bucking” slightly at any red light stop. Engine light came back on, P0171, P0174. This morning I attempt to drive to work, and she is spitting and sputtering terribly during any kind of acceleration, but idles fine. Now all 4 codes are back. Could this be a bad coil pak? Or a huge timing issue? Im to the point where Im tired of throwing money at it, and still having the same problems. Thanks for any help in advance!

  158. dennisb - Auto Tool Sales :

    Date: April 9, 2013 @ 7:15 am

    Dan, the purpose of this article was to try to save people the expense of replacing O-2 sensors when all the sensors are doing is reading the level of Oxygen in the exhaust accurately. Again, the lean codes are indicating too much Oxygen in the exhaust detected. Most of the time this means there is a vacuum leak. In some instances it could be a fuel pump starting to fail. A fuel pressure test when the problem is occurring would make sense. I would also check with an Evaporative smoke machine to find the source of the vacuum leak. The misfire code is likely to be a coil. The codes listed first are usually the worst offenders and sometimes can cause the other codes. But sense only one cylinder seems to be affected I would think the coil might be bad providing compression is normal. Good Luck.

  159. DanC :

    Date: April 9, 2013 @ 7:57 am

    Thanks, I will check fuel pressure, and if that’s fine, Ill try swapping injectors and see if the code moves. I searched everywhere for vacuum leaks, tried the old “carb cleaner” trick, every vacuum hose and fitting, couldnt find anything. I didnt say it clearly, but all those items were replaced, along with fuel filter, after the codes showed up the first time. I appreciate the help!!

  160. dennisb - Auto Tool Sales :

    Date: April 9, 2013 @ 10:49 am

    The carb spray method may not work if the leak is slow. The computer may adjust the air/fuel mixture quickly making a change in idle hard to notice. Also the vacuum leak may be larger when the vehicle is cold and smaller when the engine heats up to normal operating temp. That’s why I like the smoke test which can be performed on a cold engine. I like the low tech method test of swapping the injectors to see if the code moves. Here’s a copy of an article I wrote a couple of years ago on that topic.

    How to Check an Ignition Coil Or Fuel Injector – DIY Low Tech Method
    An ignition coil can be OHM checked with a meter to see if it is out of range or if it has a completely open circuit. But what if the car seems to run fine and a misfire code is directing you to a particular cylinder? For instance if you have a code for * P0301 (#1 misfire code) but the engine seems to be running OK, below is a low tech method for testing the coil without any special tools. This method works well for ignition coils or fuel injectors that may have an intermittent problem. Also it works best with ignition coils and injectors that are easily accessible. Obviously it’s best to check the basics first like a visual inspection to see if engine oil may be shorting a connection or if there is evidence of tracking of the spark arcing to the engine. Sometimes after an engine is cleaned water can go into the spark plug recess and short the connection to the plug as well. The simple low tech method is assuming the spark plugs and wires are good and there is still a misfire code setting.

    What is the low tech method?

    Simple, switch the coil or injector with the neighboring one. After making the switch clear the code and see if the misfire code sets for the other cylinder. Sometimes in service manuals, you may read “install a known good component.” This is basically what is being done without the expense of purchasing a new part for testing purposes. Usually with electronic parts, stores will not allow returns. This way no unneeded parts will be purchased.

    Save money by using this simple low tech diagnosis method.
    Pinpoint intermittent problems with ignition coils or fuel injectors.
    Use this method without extensive knowledge of the ignition or FI systems.
    Perfect DIY test to verify a failing component without spending money or guessing.
    By switching cylinder #1 and #2 coils the code should move from * P0301 to P0302 if the coil is bad.
    Eliminate the possibility of bad wiring or a computer driver issue.
    If the car is noticeably running rough, an ignition spark tester like the ones found at DenLors are good for an initial test. Use an automotive scan tool to check for codes and to clear the code after making the switch.

  161. Pat B :

    Date: April 11, 2013 @ 3:36 pm

    I just purchased a 97 Ford Expedition and the check engine light was on. I got the code checked at Autozone and lean on both banks. I found this forum and yes my PCV valve hose was shot. I lucky to find another big problem that I did not see covered in the discussions. I took the entire air filter/duct plastic assembly apart up to the throttle body (metal) intake opening. I found that someone had taken it off and reinstalled it with the lower rubber coupling bent into the plastic assembly rather than around the metal intake. From the top it looked OK. Once I took it off and saw the bottom section folded in… I knew where a big leak was. The throttle intake was pretty dirty and I cleaned it with walmart carb cleaner. I didn’t overdue the cleaning because I heard somewhere that too much carb cleaner can do damage. I disconnected the battery to reset the computer. After replacing the PCV and correcting the intake problem, I took the car for a long test drive and the check engine light did NOT come back on. I’m really thankful for this forum to quickly solve this problem. Now I’m worried about the dirt that may have entered the engine over ?? period of use. I added some fuel injector cleaner as a inexpensive precaution. I just bought the car, so I will see how it goes. I’m really happy the code is cleared!

  162. Pat B :

    Date: April 20, 2013 @ 9:54 am

    I was happy until my CEL came back on again with the same lean on both banks code… a day later. I have replaced every large vacuum line, but not the small plastic ones.
    Since the engine had been running without the air intake attached to the throttle body correctly, I decided to take the throttle body off completely to inspect/cleaning. I found that the ports to the EGR valve were totally clogged with carbon buildup and had to be routed out with a screw driver. The other ports were dirty but open. I wonder if I should replace the EGR valve?
    If the CEL comes on again with the same codes, I suspect my problem is the Mass Airflow Sensor. I also need to check if I reinstalled the TPS correctly… I regret taking it off

  163. dennisb - Auto Tool Sales :

    Date: April 20, 2013 @ 10:04 am

    Sounds like there is still a vacuum leak. A smoke test would be the next step in my opinion. There’s also an article on the older Expeditions with clogged EGR ports in the throttle body which causes (click the following) EGR low flow P0401 code.

  164. Pat B :

    Date: May 1, 2013 @ 2:27 pm

    Dennis, It’s been awhile and I don’t want to jinx it, but the CEl has not come back on. I applied a very thin gasket sealer to the throttle body assembly when I mounted back onto the intake. The engine idles and runs great. I really appreciate this forum and the ideas, advice and help you provide. I like to do things myself and I’m sure I saved a lot of money not buying other sensors based on the info from this forum.

  165. dennisb - Auto Tool Sales :

    Date: May 2, 2013 @ 8:31 am

    Pat, that’s great. Keep us in mind for any other repair articles of interest and of course future tool needs as they may arise.

  166. Roxy :

    Date: May 7, 2013 @ 12:21 am

    Hi Dennis,
    I have a code p0171 on toyota celica gt 2002. My car has 251,000 miles. I change my mass air flow ,oxygen sensor, manifold gasket pcv valve and also use smoke machine but still car has this code. What else do you think that I need to check. System lean is the one I need to fix.

  167. dennisb - Auto Tool Sales :

    Date: May 7, 2013 @ 8:02 am

    Roxy, If there are any other codes they need to be taken into consideration also. A vacuum leak is still the most likely cause of a lean code. A leak in the rubber air intake boot past the MASS Air Flow sensor could cause a lean code. And another possible cause is a small exhaust leak before the Oxygen sensors (leaks past the last O-2 sensor may be loud but won’t cause a code). I once found a very small exhaust leak on an Oldsmobile, at a flanged exhaust pipe connection. I was able to re-position the pipes and tighten the nuts on the exhaust studs sealing the leak and eliminating the code from coming back. A common exhaust leak on 4 cylinder Toyota’s, is the exhaust manifold itself (which bolts to the cylinder head). Gaskets can leak but I remember quite a few Toyota exhaust manifolds cracking and leaking. They tend to become quieter as they warm up, because the gap is lessened due to heat expansion. Smoke can be used to check for small exhaust leaks too. I would use a smoke machine to check for vacuum and exhaust leaks when the vehicle is cold. Leaving the car overnight with a repair shop is a good idea, so they can check these things first thing in the morning before the engine and exhaust warms up and potentially seal the leaks temporarily making detection more difficult. Good Luck.

  168. a.man1302 :

    Date: May 9, 2013 @ 3:24 am

    Hello, first and foremost I would like to thank you for this forum…it is very helpful and informative. I recently had the P0171 (bank1) popped up on my 1998 Ford expedition which has the 5.4 liter V8 engine…The symptoms that accompanied this DTC were rough riding and extreme lack of power. After checking for vacuum leaks and the MAF, I decided to check the fuel injectors and BINGO!!! They were clogged pretty bad. All the injectors on bank1 looked as if they were jammed with ear wax. After cleaning them I drive the car to my cousins and surprise surprise the check engine light popped up again this time with P0174(bank2). Checked injectors and they were also clogged. After a week of driving no more check engine light. I will be changing the fuel filter as soon as I have time…oh yeah I forgot to mention that I changed all the spark plugs since I had the fuel line off and the injectors out. The last two spark plugs on both sides are a real PAIN!!!

  169. dennisb - Auto Tool Sales :

    Date: May 9, 2013 @ 3:04 pm

    Thanks for the comment. Clogged injectors can occasionally be a problem that can cause lean codes because there’s more oxygen making it to the oxygen sensors (because the fuel is not delivered properly which normally uses oxygen when ignited). Problems can occur, especially in vehicles that have been sitting for awhile. Gasoline with just 10% ethanol can deteriorate after a couple of months, which can cause deposits and gumming that can clog fuel injectors. That’s why using a fuel stabilizer is so important if a vehicle is going to be stored or just not driven for extended periods of time. Thanks again for commenting.

  170. Jeaton1985 :

    Date: June 14, 2013 @ 6:21 pm

    Ok I have a 94 ford F150 with a 5.0 just rebuilt the entire engine and have replaced many parts fuel pump, gas tank, fuel filter , gear box, PVC valve, thermostat, ECT sensor, plugs, wires,rotor,cap,even more I’m sure I’m forgetting also I have repaired many vacuum hoses I have used a smoke machine and cannot find any leaks, I hooked the truck to a scope and saw misfire on different cylinders periodically also scoped my O2 sensor and found it to be running lean it has a rough idle and you can feel it missing opened the air filter box and sprayed in propane to richen the mixture truck smoothed right out and idled great while doing so indicating it is for sure running lean only code it has and not always is a 211 pip erratic scoped pip signal square wave looked good cant figure this out . I was told my MAF may be out of range and causing this problem I have taken it off and cleaned it with no change did notice one of the resistors in it was a little swollen please help. Also truck only has 524 miles on it.

  171. dennisb - Auto Tool Sales :

    Date: June 16, 2013 @ 9:44 am

    The 211 Profile Ignition Pick-up circuit fault could be causing the stumble. The computer may not always be seeing the PIP signal generated by the Crank Position Sensor (CKP). I would see if there’s a break in the circuit for the PIP and check a repair manual for specific test procedures. The MAF could be replaced also since there’s an obvious sign of a bad resistor. Good Luck.

  172. Fordever :

    Date: June 24, 2013 @ 11:07 pm

    Dennis, you seem to be the one who could diagnose this nightmare. 2001 f150 4.2. Of course p0171 and p0174… I bought the truck with the light on and figured it wouldn’t be much to fix. I reset the computer and the light stayed off for about 1000 miles then came back on. When I start it cold it will go to about 1500 rpm and take about a minute to get back down to 800… and about halfway warmed up it would start to idle rough but if I give it any gas at all it never hesitates, and will idle normal after a couple seconds of holding it at about 1500. And even when it’s hot it will sometimes idle rough at a stop. While hooked up to a scanner the short fuel trim fluctuated pretty low at some points… Can’t find a vacuum leak but haven’t smoked it. Cleaned IAC new intake MAS cleaned… Is anything I’m saying pointing towards maybe a fuel pump or sending unit?

  173. dennisb - Auto Tool Sales :

    Date: June 25, 2013 @ 7:41 am

    Most common cause of “lean codes” is still a vacuum leak. I would visually check the PCV elbow. Make sure there’s no exhaust leaks. Verify there’s no cold air leaks past the Mass Air Flow Sensor. And have the rest checked with smoke when cold. Since it takes awhile for the the code to be triggered; Could be a cold leak from the intake gaskets (seals), the manifold bolt seals (isolators) or the IMRC (Intake Manifold Runner Control). Fuel pumps and regulators rarely cause these codes, but the fuel pressure can be checked easily just to rule low fuel pressure out as a potential cause. The fuel sending unit has nothing to do with fuel delivery or pressure. It just signals the fuel gauge how much fuel is in the tank. Good Luck.

  174. Fordever :

    Date: June 25, 2013 @ 1:57 pm

    When I first got it it took a while for the code to return now I can’t keep it off

  175. dennisb - Auto Tool Sales :

    Date: June 25, 2013 @ 2:03 pm

    Leaks only get worse…

  176. kdeh2 :

    Date: July 20, 2013 @ 1:56 pm

    Help! I have a 1996 Jeep Grand Cherokee that stalls intermittently. I have replaced parts on it for a year now and still it is still stalling, especially at stop lights or upon acceleration after sitting at a stop light.
    The following have been replaced to try and fix it:
    PCM
    Crankshaft Sensor
    Throttle Position Sensor
    Catalytic convertor
    Fuel Pump
    Also replaced:
    New Muffler -after it backfired and blew hole in old one
    New Tail Pipe
    New Serpentine Belt
    New Water Pump
    New Radiator
    I know it has to be something simple but unable to figure it out, even professional mechanics are just guessing, thus the reason all the above parts being replaced. The most recent was the PCM, which after being installed it ran fine for a few days or so, then started all over again. The codes have been random, but now, using the key On/Off, On/Off, On/Off, On method no codes showing other than the code 12 (battery has been disconnected). Using the key On/Off, On/Off, On/Off, On method.

    I ask you because after reading this article, you seem to be a true expert and someone opinion to I can trust. Do you think it’s possible that it could be a vacuum leak causing it to stall? It seems this is a very very common problem with these jeeps.

  177. dennisb - Auto Tool Sales :

    Date: July 20, 2013 @ 8:06 pm

    First, not all the things on the list were done for a stalling issue. The PCM, Crankshaft Sensor and the fuel pump are the only things that were replaced that could cause an engine to shut off.

    The PCM hardly ever goes bad but is commonly blamed whenever nothing else can be pinned down… must be the computer!

    The CKP (Crankshaft Position Sensor) is actually a pretty common failure on the Jeep. There’s not always a code set either when it fails. It counts revolutions of the engine’s crankshaft. So when it fails, especially intermittently the computer just thinks the engine is not turning. Therefore doesn’t provide spark and fuel etc.

    The fuel pump is a fairly common failure too. Slow to start when cranking or intermittently shutting off is a common symptom, the fuel pump can be double checked with a fuel pressure gauge.

    Vacuum leaks are not all that common on Jeep Grand Cherokees (unlike with Ford products).

    Any CEL (Check Engine Light) codes are always the best bet to find out what’s going on.

    With no codes present, but the stalling persisting usually at stops I would suspect a carboned-up IAC (Idle Air Control) motor and/or dirty throttle plates. I’d also have a load test done on the battery just to make sure it’s not marginal. Sometimes when a battery is going bad, enough voltage is not provided for the computer’s memory to be maintained, resulting in the fuel strategy being lost. This causes it to stall until it’s fuel strategy has been relearned again. It may take even longer if the throttle plates are really dirty because, the carbon build-up affects the volume of air entering the engine. While checking the battery, make sure all connections are free of any corrosion and that the terminals are tight. Good Luck.

  178. SLKgirl :

    Date: July 26, 2013 @ 10:59 pm

    Vehicle: 2012 Mercedes SLK 250
    Mileage: 27,000 (bought new from dealer)
    Maint. History: had regular 10k, 20k mile service
    Fuel: 60% of the time 91 octane, 40% of the time 100 octane
    Always use at 91 octane Chevron, rarely use 76Station
    100 octane is always the race fuel from 76
    Engine mod: K&N air filter only out of the box done at 22k miles

    Symptoms: CEL comes on after hard acceleration in 2nd gear- using paddle shifters – always happens from standing start in 1st gear. When this happens, tach is at approx 4500 then the car lurches, w power loss – suspect its cylinder misfire. It’s corrected itself before – tried re-seating fuel cap. Tried turning engine off then back on. Tried turning engine off, took key out of ignition, locked doors, unlocked doors, turned engine on. All of these cleared the CEL at one time or another but it’s very random. Currently have tried all these again and nothing didn’t clear the CEL.

    Wondering what is causing this problem. Have seen the same problem on SLK forum on different years, anywhere from 1999-2013 including 2013 SLK55 AMG. The owners have taken their vehicles to the dealer and the older ones were corrected from replacing coils or cleaning injectors. This problem is still not diagnosed by the dealer in newer models. The one person w a 2013 SLK55 AMG still has his car at the dealer – it’s been there for 3 wks already and they haven’t figured it out.

    Any thoughts on what my issue is? Thanks in advance for your help.

  179. SLKgirl :

    Date: July 26, 2013 @ 11:04 pm

    Addendum: I drive my SLK pretty hard – no Sunday driver type driving – this problem started happening when I passed 8000 miles.

  180. dennisb - Auto Tool Sales :

    Date: July 27, 2013 @ 3:42 pm

    Obviously the vehicle is under warranty. K&N filters may cause these lean codes, but the dealer won’t warranty anything related to aftermarket parts (even though you may have had the problem prior). Turning the engine off and locking/unlocking the doors has nothing to do with the power train control system. So if the CEL went off after doing any of those things it was purely coincidence. I don’t have any other suggestions, I would take it to the dealer for warranty repairs, but feel free to check back in case anyone else has input. Good Luck.

  181. SLKgirl :

    Date: July 27, 2013 @ 10:38 pm

    Thx for the feedback – actually I’ve been having the problem prior to putting the air filter in. It may be related to some sort if vacuum issue. The CEL was on + the power loss. I parked the car and ran an errand for 1 hr or so. The CEL was still on when I restarted the car. I tried reseating the fuel cap but the light remained on. The power returned though. Left the car parked overnight. In the morning I started the car again. CEL was still on. Reseated the cap again. Restarted again and light didn’t come on. Is it possible the change between 91 octane fuel to 100 is a problem? Prior to this I put a full tank of 100 in my car. My car was also almost on empty when I re fueled. My computer told me I had 28 miles remaining on the remaining fuel before refill.

  182. dennisb - Auto Tool Sales :

    Date: July 28, 2013 @ 8:56 am

    Since the vehicle is under warranty, I would definitely let the the dealer take care of this issue. It’s not common to have vacuum line issues on such a new car and a loose gas cap will not cause lean codes (which is what a suppose you have since you are commenting on this article). And changing the Octane is not going to set a lean code either. It could be a software update issue and the vehicle’s ECU may need to be flashed correcting the parameters allowed without setting a code. The dealer is best equipped to determine this. Good Luck.

  183. SLKgirl :

    Date: July 29, 2013 @ 2:50 pm

    Thx I’ll have them re-flash the ECU or at least check it out. I’ll let you know the outcome.

  184. Nick :

    Date: August 4, 2013 @ 1:37 am

    I’ve got a new focus ST, I’ve got 14k miles on if. It just started intermittently dying at idle or with the clutch in. Finally after 10-15 times over the course of 3 days it through p0171. I haven’t been able to get to the dealer (it’s the weekend) but just curious your thoughts. From what I’m gathering, check all vacuum lines, MAF Sensor and O2 sensors. I have a Cobb access port, which reflashes the ECU, and is able to monitor all sensors live so I plan on logging what they’re doing. Also, the car is doing it with stock and reflashed ECU, I’ve gone back and forth thinking it might be the tune but it doesn’t seem to be. I also read on another forum STs getting the idle problem without ever installing the tune, so I don’t think that’s it. Thoughts??

  185. dennisb - Auto Tool Sales :

    Date: August 4, 2013 @ 8:48 am

    Since you say the car still has the issue with the OE ECU, that eliminates the problem being the aftermarket one. The car is too new for me to suspect an intake gasket or vacuum hose leak. I would look around the cold air intake hose for any loose clamps or wrongly positioned hoses from the air filter box all the way to the throttle body. Any small leaks past the MAF (Mass Air Flow) and before the throttle body could cause stalling and a lean code. I would have someone push the clutch pedal in while I’m looking under the hood also, to see if any interference from the linkage could be rubbing on a hose causing the problem. Good Luck.

  186. BobC :

    Date: August 7, 2013 @ 5:38 am

    1998 Jeep Cherokee Classic 4.0
    2wd AT
    Jeep 4.0 Engine 116,000 mi
    Had it 3 yrs and it burned/melted a hole on edge by spark plug. of #2 piston.Was told it was from lean burn on that injector dont know for sure.
    Bought a used engine with 117,000 mi had it installed by good mechanic.
    Left everything under hood that was there originally. Never found out what caused the piston burn.
    The newly installed engine was hesitating today, bogging down when accelerated until it got up to 2000 rpms then ran great unless it got below 2000rpms. So put new plugs in (type that came in it from factory)set the gaps .035 and bingo eliminated that problem. Sometimes when cold or hot it will still hesitate just a little. Cleaned throttle body good checked inside dist. no carbon tracks or cracks or moisture visible.
    Problem: Was going 55 mph for 4 mi. started surging and cel started flashing.Stopped and turn engine off. Oil level good,had just backflushed cooling system. filled with Peak 50/50 coolant with front elevated to burp the air out. It was not running hot at all a little less than 210 degrees. Returned home (4mi)flashing cel did not flash anymore but stayed on. Had the mech. come over and put a Noid light on the injectors they all checked as working. His code reader read PO301 Cylinder misfire detected-cylinder number 1. Pulled # 1 wire off with running engine and it made a difference so it was firing did other 5 with same results. Also pulled all 6 injector plugs one at a time and it made a difference on each. He Told me to put new plug wires and distributor cap on (wires are old (orange) going to do that tomorrow.
    Bottom line is I am gun shy, so afraid same will happen to this engine that did the prior one, that I wont drive it far. Is there anything else that would cause it to happen again from what I’ve said? Other things to look out for that may not throw a code? Oh I will check for vacuum leaks too.
    Thx for all you do !!
    BobC in NC

  187. dennisb - Auto Tool Sales :

    Date: August 7, 2013 @ 7:43 am

    Bob, hopefully you have had the compression checked on the “new to you” engine that has 117,000 miles on it. Most salvage yards will offer a limited warranty, in case you get one with a problem. I would just pay attention and address any CEL’s that come on. The previous engine more than likely was driven with a trouble code for quite some time. I say this because a lean condition (probably from a clogged injector) that was affecting ONE cylinder severely enough to melt the piston most likely was driven with a CEL on for a long time. This doesn’t happen overnight.

    So to recap. Mainly I would make sure there’s no trouble codes. If the CEL comes on, pay attention and fix it quickly. A flashing CEL means that there’s a bad bad misfire. So bad that it will cause damage to the Catalytic converter quickly because of excess un-burned fuel. Good Luck.

  188. Kisler :

    Date: September 27, 2013 @ 7:33 pm

    I have a 2001 Ford Expedition. I’ve know for a while that I had a vacuum leak because I took it to the auto parts store and they diagnosed the problem. I didn’t fix it right away but yesterday it started dying when I was Idling so I took it to the repair shop. They told me that they used a smoke machine to find the leak (it was leaking in three places). They also said that it was reading lean on both sides. They replaced the hose and the sensors too. It cost me $340.00. I’m just wondering if this was a fair price for the work that they did. I’m a single woman and I have no way of knowing if I was taken to the cleaners or not.

  189. dennisb - Auto Tool Sales :

    Date: September 28, 2013 @ 8:38 am

    Kisler, like my article says in the first sentence “If a car has a check engine light on and has P0171 and P0174 codes stored, the most likely cause is a vacuum leak.” That means that we will never know if the Oxygen sensors really needed to be replaced or not. They could have been reading accurately, picking up the extra oxygen in the exhaust due to the vacuum leak from the hoses (that’s very common). The way to repair the vehicle the least expensive way would have been to fix the vacuum leak and clear the codes. After driving a while if the codes came back and there was no vacuum leak, then I would look at the O-2 sensors again. If the codes didn’t come back then you would have verified that it was just the hoses. With that being said your vehicle (at this time) is 12 years old. So if the sensors were never replaced before, they may have failed soon anyway. As far as the cost that you paid for the work an parts installed; you should have received a breakdown of labor and parts. You could check with another shop to see if the price was comparable. But it’s not really time to comparison shop after the fact. The time to find a repair shop that you trust is before you need one.

  190. Kisler :

    Date: September 29, 2013 @ 8:03 am

    Thank you so much for the information. Regardless of the price I paid, I’m on the road and the sensors had never been changed, so either way, I feel that I’m better off with everything having been done so that I don’t have to face it in the future. I’m so happy I found your site! Thanks again

  191. dennisb - Auto Tool Sales :

    Date: September 29, 2013 @ 9:43 am

    Kisler, glad you found us too. We offer free advice on automotive repairs. We appreciate when our readers need auto tools, that they keep us in mind at DenLorsTools.com. Also, Like us on Facebook for special deals and check out our YouTube Channel for more free tips on repairs and car tools. Have a great weekend.

  192. kanders :

    Date: October 25, 2013 @ 12:22 am

    Wondering if you can help with a related issue. Have a 97 Ford Ranger (4.0) that throws p0171 & p0174 codes. Replaced MAF, PCV and elbow, fuel filter, cleaned throttle body and replace upper plenum gasket. Mechanic also did smoke test with no obvious issues. In colder weather, after truck has warmed up, engine will surge around 500 – 800 rpms. If I coast down the road, the rpms drop to 500 rpm and quickly surges to 800 and quickly repeats. Any help is appreciated. Thanks.

  193. dennisb - Auto Tool Sales :

    Date: October 25, 2013 @ 8:20 am

    All indications are there is a vacuum leak. I would check the lower intake gaskets with Fuel Injector spray. It may idle up if the leak is large enough. If still cannot pinpoint, maybe let another shop check in case your mechanic is overlooking the cause. He may not want to admit the lower intake is leaking, after he replaced just the upper seals. Good Luck.

  194. kanders :

    Date: October 26, 2013 @ 6:47 pm

    Thank you for your suggestions. I’ll try the fuel injector spray and then take it to another mechanic. I’ll let you know how it goes. Thanks again.

  195. Shiro :

    Date: November 18, 2013 @ 4:48 am

    Can you help,
    i have 97 sebring convertible, cold start idles fine, after warm up and driving for about 20 mins then shut off the car about 10 mins later when i start, it idles terrible, sometimes it sputters and shuts off.
    had multiple cylinder misfire codes and lean on bank 1 codes. when it sputters and i press on the gas pedal, there are sounds like banging, like backfire.

    changed in order, MAP sensor, intake manifold gasket, EGR valve, spark plugs and wires, IAC valve, upstream o2 sensor, PCV valve. There is fuel pressure.

    after all these, the code P0171 still pops up. when driving it is perfect, at red lights it idles fine. Only starting has problem and sometimes it wont start, until i spray the carb cleaner, and open the throttle and rev a few times, it starts idling properly……

    whats wrong with my car?

  196. dennisb - Auto Tool Sales :

    Date: November 18, 2013 @ 7:25 am

    Regardless of the work previously done, the P0171 indicates a LEAN CODE. 1. A lean code is usually caused by a vacuum leak, like an intake gasket or vacuum hose. If the intake gasket was installed incorrectly the gasket may not be in the right place allowing a leak. I would glue them into place if there was a doubt about it staying in position while installing the intake 2. If there’s an exhaust leak, even a very small one (before an Oxygen sensor) the code could also be triggered. The outside air skews the readings since it has more oxygen than the air that’s burned by going through the engine. Exhaust leaks past the O-2 sensors will not cause a lean code. 3. Also low fuel pressure can be the cause but this is not a very common cause of a lean code. If everything else checks out OK, I would check the pressure readings and compare to the manufacturer’s specifications. Just because there is some pressure, doesn’t mean there is enough to be within the correct parameter.

  197. Steven :

    Date: November 18, 2013 @ 11:30 am

    I have an 01 F150 5.4 Been having problems a few weeks now and I know there are many different things that can cause the po171 and po174 codes. I have not heard any hissing while hot or cold, took to shop and they also heard nothing hot or cold. Also performed smoke test while cold, no leaks. What would be your next suggestion?

  198. dennisb - Auto Tool Sales :

    Date: November 18, 2013 @ 12:24 pm

    You won’t hear hissing unless the leak is large. The smoke test, if done properly is very effective. The most common leak on some years of the F150 is the PCV valve hose. More specifically the elbow (90 degree part). Even if the elbow is collapsed, it can set the code. Also on some years of the F150 which have a Mass Air Flow sensor, a loose clamp or a tear in the fresh air intake hose (which is in between the sensor and the throttle body) can allow unmetered air to enter the engine. The unmetered air can cause the computer to set the code because there’s more Oxygen than expected or calculated. Sometimes a dirty Mass Air Flow sensor can cause lean or rich codes. I would inspect the air filter for dirt or tears along the edges which seal and force air through the filter rather than around it. Again, exhaust leaks can confuse the computer. The sensors are detecting Oxygen, so if there’s an exhaust leak which can allow fresh air to enter, the reading will be too high appearing lean, setting a lean code.

  199. Shiro :

    Date: November 18, 2013 @ 7:44 pm

    Thanks, will look for leak tonight and keep you posted….

  200. Luke :

    Date: November 26, 2013 @ 1:26 am

    Hey! Great site. Sorry to hear so may other people have the same OBD codes as me.
    I have a 1997 GMC Yukon SLT, 5.7 liter.
    I paid $400 to replace the lower intake gasket about 2 years ago after I spotted an antifreeze leak. Since then I have been through about three sets of plugs, wires, caps and rotors; which seem to fix a chronic misfiring problem for a few hundred miles and then the misfires return. Used to drive smooth for about 100 miles, then would miss for 50 miles, and so on. Now seems to be misfiring really bad more often and exhaust is popping under car (a recent symptom)
    The codes I get are P0300, P0171 and P0174. I also began getting the rear cat converter efficiency below threshold after problems developed on freeway and the CEL would blink when accelerating under load. Replaced cap and rotor after that and things were fine for a while.
    Seems to be worse when damp, or after I restart the car after a short trip.
    Sprayed carb cleaner around manifold and plenum gaskets and didn’t notice anything. Parts guy recommended spider injector. I was thinking O2 sensor as an easier thing to try first, but afraid of throwing money at parts without really knowing what the problem is.
    Recently had it hooked up to a friend’s $1,000 scanner which showed 10,000-plus misfires logged on every cylinder over the last 90 or so keystrokes. Took electrical connection off injector and everything looked fine.
    Help, and thanks in advance…

  201. dennisb - Auto Tool Sales :

    Date: November 26, 2013 @ 8:18 am

    Luke, The P0300 is a random misfire code. The P0171 and P0174 are lean codes. If the random misfire (P0300) started right after the intake gasket was installed, it’s possible the distributor was not re-installed in the correct position. The distributor has to come out when removing the intake. It’s a common mistake to put it in wrong when going back together with the job. Spark plug wires caps and rotors don’t need to be changed as often as you’ve said. They simply don’t wear out that quickly. However, if the spark plugs are fouled and show a lot ash build up, the engine could be burning excessive oil (from worn piston rings or valves). You should know by the amount of oil you have to add between oil changes. Whatever the amount, if it’s not leaking it, it’s burning it. The lean codes P0171 and P0174 could be from and exhaust leak. This makes sense because you say you’ve heard one, the exhaust popping. The catalytic converter efficiency code below threshold has nothing to do with the cap and rotor. It means the catalytic converter is not working enough to burn the unused fuel before the exhaust goes out the pipe. The catalytic converter most likely is getting restricted also, which could force an exhaust leak to occur. In short, sounds like several things could be going on. But based on the age of the vehicle and the symptoms, the engine most likely is burning oil causing plugs to foul and misfire. And the oil has finished off the catalytic converter, which was due to expire anyway being 16 years old (if original).

    BTW, you mentioned Keystrokes? I know what you meant, but never heard it called that before. I think of keystrokes in relation to keyboards (computers, pianos etc.) not vehicle ignition keys and start ups. Anyway, Good Luck.

  202. Luke :

    Date: November 26, 2013 @ 2:14 pm

    Thanks for your advice, Dennis. A mechanic friend this morning said sounds like I have a plugged cat. Like I said, it could be just about anything–only problem is where to start. Might just break down and take it to the Chevy dealer for a diagnosis. At $80 per hour, could get pricey real fast. Doesn’t seem to be burning oil, but I do have a rear main seal leak drip (a few spots on the garage floor each day) that requires me to add about a quart of oil every 2-3 months or so.Other garages keep wanting to change plugs, wires, caps and rotors; but that has been done so much without longterm success that the problem is definitely someplace else.

  203. Travis H :

    Date: December 6, 2013 @ 11:51 pm

    I have 1999 ford explorer 5 speed manual im having problem idleing bad spark knock while at idle an sometime it wont idle I can cruse 45 come up to a stop push in clutch an rev up to 2000 to 2500 an stay there till depress the clutch but ive clean mass air flow, change fuel fuel filter, got perfect fuel pressure change plugs and wires an gap is correct on them, im getting lean code on bank 1 an 2 ive checked for vacuum with carb cleaner and with the propane I don’t have a smoke machine that’s only thing that I haven’t tried just wondering if you have ever seen something similar Im running out of ideas

  204. dennisb - Auto Tool Sales :

    Date: December 7, 2013 @ 7:04 am

    Travis, it sounds like a vacuum leak. The most common leak on the Explorer is the intake gasket seals. Good Luck.

  205. Luke :

    Date: December 25, 2013 @ 9:18 pm

    After months of investigating, I think my misfire has been solved on my 97 Yukon. I I had been getting P0300 codes and the check engine light was always on. Well, my mechanic said he adjusted the distributor cam sensor timing to crank -2 degrees. No more misfire and P0300 code gone. The mechanic who replaced my intake manifold gasket a year or so ago may not have put the distributor back in right. I am still getting a code for inefficient rear passenger-side cat, but I think that might be the rear O2 sensor on that side or the cat has reached its end. But, I am happy to report the misfire code is gone and the truck is running great…of course one week after getting this solved, the fuel pump went out.
    Thanks for your advice.

  206. dennisb - Auto Tool Sales :

    Date: December 26, 2013 @ 6:30 am

    Luke, glad I could help. Keep us in mind for your future tool needs and we appreciate likes on Facebook too. https://www.facebook.com/DenLorsTools

  207. Horizon :

    Date: January 2, 2014 @ 8:59 pm

    2006 ford tarus with 3.0
    Has p0171,p0174 and p0300
    MAF reading at idle 3G/S
    R/R MAF
    Codes back
    Any help hard to detect any vacuum leak

  208. dennisb - Auto Tool Sales :

    Date: January 3, 2014 @ 8:36 am

    Could be an intake runner leak on a Taurus. See the following article if your intake has runners like the Ford Windstar.
    Intake Runner Vacuum Leaks

  209. terri :

    Date: January 15, 2014 @ 10:27 am

    replaced o2 sensors, egr sensor and valve and vacuum hoses and still have the check engine light on.. 1999 ford taurus.

  210. dennisb - Auto Tool Sales :

    Date: January 15, 2014 @ 10:43 am

    There are many different codes possible… I would check the vehicle depending on which codes are stored.

  211. Dan :

    Date: January 17, 2014 @ 1:25 pm

    First off I wanted to thank you Dennis for following up for years on this article, I have been having the same 2 lean bank codes on and off for a month or so now. Fuel trims saw a rise as much as 20% or so while maintaining a cruising speed, seems to return with the +/-5% range when 3/4-full throttle is applied. I have checked for vacuum leaks, everything seems good. After I cleaned the MAF sensor and had a mechanic reset the learning parameters (or something like that), the car immediately sounded better,felt much smoother, and pulled much harder. It just felt healthier so to speak. This seemed to cure the problem for me and everything was great for about a month. When I started and drove my car yesterday I felt the car feel slightly weaker, and the note in the exhaust slightly changed (like before). sure enough when I started it this morning the codes returned. Could this mean more than likely a bad MAF? I mean after cleaning it, it cured all the problems from power, to sound, and smoothness in throttle response for about a month. Hoping you could shed some light for me. Thanks again!

  212. Dan :

    Date: January 17, 2014 @ 1:31 pm

    My apolgies for a double post, but I was also wondering as a temporary measure to prevent any damage, whether or not I should unplug the MAF sensor and let the car use normal fuel parameters while driving until I can figure out the root cause?

  213. dennisb - Auto Tool Sales :

    Date: January 17, 2014 @ 1:47 pm

    Dan, usually these codes mean a vacuum leak. I know you’ve looked for obvious vacuum leaks, but sometimes they are very difficult to locate. Sometimes the leak is only present during either cold or warmer temps. I would smoke test (or have a shop use a smoke machine) to locate the vacuum leak. An exhaust leak could be the problem too. A leak in the rubber (or plastic boot) could cause the codes. The reason it seemed OK for awhile may not have been the cleaning of the MAF, it could have been from clearing the code and the fuel strategy going back to normal until whatever the cause of the problem altered that again. It takes several”trips” for the computer to do a self test of all the systems and trigger a code if there’s something that failed. As far as unplugging the MAF temporarily; usually cars will run and idle with the MAF unplugged but as soon as you apply the accelerator the engine will shut off.

  214. Dan :

    Date: January 17, 2014 @ 5:00 pm

    I appreciate the quick reply. I will have it smoke tested tomorrow morning while its cold, in the meantime Im going to reest the fuel trim and memory to get the car running smooth again at least temporarily. I will post a follow up tomorrow, thanks!

  215. Rick :

    Date: January 21, 2014 @ 8:19 pm

    I have a 1997 F150 with 4.6 ltr engine. I am getting one code and this one code only P0171. In my estimation this will eliminate most air leaks behind the MAF, a leaky gasket between the intake manifold and the TB, a leaky gasket between the TB and the EGR, bad fuel filter or pump and other components like the DPFE etc. because all of these would affect both banks. Correct me if I am wrong. But my problem is reserved for bank #1 (passenger side). While the engine is running I sprayed starter fluid around bank #1 intakes and there was no change in engine rpm. So if there is still a leak somewhere, where would it be? What is my next move to check? All vacuum hose rubber pieces that go to the TB have been replaced. However there is one hose that goes from the back of the TB to the firewall. Where does this vacuum line go to?

  216. dennisb - Auto Tool Sales :

    Date: January 22, 2014 @ 7:36 am

    A vacuum leak on the intake can affect only one bank. A small exhaust leak before the second Oxygen sensor can affect one side as well. Spraying starting fluid near a vacuum leak does not always make the RPM’s change on a modern engine. The computer can compensate the fuel trim rather quickly making this test not too effective if there’s only a small leak. You can look at the fuel trim when spraying starting fluid near a vacuum leak but there could be a little lag (delay). Sometimes you can look on the white sticker under the hood for some vacuum hose routing, otherwise I suggest looking in an auto repair manual or go to the parts department at the dealer for help in identifying components like the hoses and where they are routed,

  217. Chris@backroadsracing :

    Date: January 28, 2014 @ 10:23 am

    I own a 1999 chevy silverado, it has irratict idling and it is really bad on ga. My friend is a mechanic, and he said that it could be any one of the sensors on my throttle body, or it could be my mass airflow sensor. So i unplugged them one by one listening to which one shut the engine down… well all did except for the mass airflow and the idle air control, i have changed the idle air control and still does same thing, i unplug and plugged in the maf sensor and there is no change in the engine. If anybody knows how i can fix this, that would be highly appreciated!

  218. dennisb - Auto Tool Sales :

    Date: January 29, 2014 @ 10:32 am

    I would check the actual gas mileage. Depending on the load, size of tires and driving habits, 12-20 miles per gallon is realistically what should be expected from 15 year old Chevy truck. Unplugging sensors is no test. Most faults will be detected by the vehicle’s computer when it runs self tests. All OBD II vehicles since 1996 are required to do these self monitoring tests which set trouble codes for most issues resulting in the CEL (Check Engine Light) to be turned on and the code stored. Unplugging sensors will most likely set all types of false codes. I would clear them all and drive the vehicle to see if any real codes are set. Good Luck.

  219. Josh :

    Date: February 11, 2014 @ 1:59 am

    My truck:2000 Ford F150 V6
    My problem:Check Engine Soon light came on after I installed the K&N filter and yea I oiled it up with the k&n oil clean kit. Truck drives fine but light shouldn’t be on. How do I fix this?

  220. dennisb - Auto Tool Sales :

    Date: February 11, 2014 @ 6:33 am

    Electronics CleanerClean the Mass Air Flow Sensor wires with an Electronics Spray Cleaner like CRC makes and clear code. If the code returns you may need to replace the MAF. I would also make sure there’s no cold air leaks before the MAF. Good Luck.

  221. steve :

    Date: February 17, 2014 @ 7:27 pm

    Hello, I have a 2000 F150 ext cab 4×4 Recently I got my front axle pumpkin replaced it sat for about a month at the repair shop. I got it back and noticed it bucking while driving brought it back got the plugs a coil and the boots changed. Now it has a vibration around 60 mph I was told squared tires so got a couple new ones but the vibration remains and a code just came up both sides are running lean according to the part store sold me injector cleaner and said maybe a o2 sensor or vac lines any suggestions?

  222. steve :

    Date: February 17, 2014 @ 7:28 pm

    That’s a 5.4 engine

  223. dennisb - Auto Tool Sales :

    Date: February 17, 2014 @ 7:35 pm

    Lean codes are usually from vacuum leaks (see the article for tips). The vibration is unrelated to the codes. Vibration while driving is usually from the condition of the tires, wheels or the wheels being out of balance. Since you’ve got new tires I would check for bent rims and wheels being out of balance. I’d make sure the new tires aren’t out of round. Good Luck.

  224. jorge g :

    Date: February 26, 2014 @ 7:32 am

    Hi Dennis I have a Ford Windstar 98 with a 3.8 Liter motor. I took it to pep boys to check with the OBD scanner and they gave me this code; PO113. I replaced the MAF and the check engine light is still giving me a headache. Hope you can give me some tips please.

  225. dennisb - Auto Tool Sales :

    Date: February 26, 2014 @ 7:49 am

    Jorge, You have an Air Temperature Sensor code that has nothing to do with the MAF. The sensor could be bad (open circuit) or there could be a broken wire. The sensor looks like the one in the picture.

  226. Cal :

    Date: February 28, 2014 @ 7:34 pm

    Hi folks,

    We now have that dreaded “light of death” on our dashboard this week, so took our ’99 Ford Explorer into a local dealership to pull the codes.

    You guessed it, after ‘inspecting the cause of the malfunction indicator lamp’ … it produced P0171 system to lean Bank 1 and P0174 system to lean Bank 2 – suspect possible fault with intake manifold – need more time to diagnose.

    So, does this mean we will have a larger bill the next time, where they re-assess the problem again???

    At least on this visit, we had them carry out an oil change (long overdue) and rotate the tyres.

    Now, we are heavily concerned that a costly repair is just around the corner.

    Worried.

  227. dennisb - Auto Tool Sales :

    Date: March 1, 2014 @ 8:31 am

    Cal, that’s par for course when driving a 15 year old Ford. I’ve replaced intake gaskets on Ford Explorer’s when they were only a few years old. At least you don’t have a car payment ;)

  228. Cal :

    Date: March 1, 2014 @ 4:30 pm

    Thanks for the reply Dennis!

    The one thing I didn’t mention, is that the truck has only 62k on the clock – very little driving in it’s years, as it was inherited from father-in-law, who mainly kept it for longer journeys and drove his car 90% of the time.

    Anyway, I guess we now drive it with that distracting engine light on – and see if any problems occur over the next while, then it’s back to the garage with more $$$’s in hand! :(

    All the best.

  229. Rosie :

    Date: March 4, 2014 @ 11:06 am

    Hi Dennis!! Your site is seriously awesome! I wonder if I could bother you for some help!! We have a 2001 Mercury Cougar, which only has 77,000 miles on it. Shortly after we bought the car new, we received a letter, which was a Technical Service Bulletin and not an official recall, stating that we should get the fuel pump replaced. It was free and it would be good for 10 years from the original purchase. Well, of course now that the 10 years has passed, we are now having trouble with our car. The garage that we usually go to changed supposedly put in new spark plugs and did a V6 tune up, about a year ago. We took it home, and it smelled like rotten eggs, took it right back. The guy said it was something they did wrong, and said he had to replace a part. He didn’t charge us, but I should have gotten it in writing what he did. It ran pretty good for 6 months or so, but with the frigid temps we have, it is hesitating and stalls once in a while. The one day it was totally dead, and it actually needed a new battery. The old one was 7 years old. It worked better for a while, but the last month it started acting up again. It seemed like it would run better if we kept the gas tank full, but now that’s not even helping. When it is idling, the tachometer bounces and sometimes it will stall. I took it to the local auto store and they did a quick check, which said “bank 1 and bank 2 system too lean”. I am taking it to a mechanic today to get the OBD codes checked on a better machine. Do you think it is a major fix or hopefully just an oxygen sensor or what?? I am just hoping it isn’t thousands of dollars, of course! Oh, I forgot to mention, I swear I hear a hissing sound under the hood…..maybe it is a simple vacuum hose leaking! Thanks for your help!!!

  230. dennisb - Auto Tool Sales :

    Date: March 4, 2014 @ 11:33 am

    Thanks Rosie. I would suspect that the codes have nothing to do with the past problems. Most likely just a vacuum leak. Good Luck.

  231. Heather :

    Date: March 6, 2014 @ 6:45 pm

    My car has a check engine light on and has P0171 and P0174 codes.

    I took it to a shop and they ran diagnostics and said I need new upper and lower intake gaskets. They didn’t have the car back there long before they told me this. I am wondering if they just guessed to get the most money out of me. I took my car to my friend that is a retired long time mechanic at a big name car company. He sprayed all around the intake gaskets and did not see any sign of a leak. He is doubtful that that is what the problem is. I was looking online and found the Smoke Test. I think I will find a place that does that and see where the leak is to make sure.

    My car runs pretty much okay. The problem is getting it started. Before the winter hit, my car would only have problems starting if I stopped at several places where I had to turn my engine off, like while out shopping. After the winter hit it does not like to start especially after sitting over night. It will start then die, and do that 2 or 3 times, then it will stay started for a few then sometimes die out. Then by the next start is stays started. But I have to let it warm up enough, or if I pull off before it is ready, when I push on the gas it will putter and shake a bit then die. Then I restart it and it is fine and good to go.

    We had a few warmer days, then it started doing the thing it does before the winter hit, again.

    I found online that for a lot of people with my described problem, changing the MAF sensor worked. Well it did not work for me. I did feel more power in my engine but it did nothing for the starting problem. Also, if I cant get my car to start at all, doesn’t happen to often, all I have to do is unplug the MAF sensor and it starts right up. Then I plug it back in and I am good to go.

    Another thing. I went through a car wash and got an under car rinse to get off the ice melt chemicals. After I pulled away my car wouldn’t go over 30mph for like a minute, then it just took off like normal.

    Have you seen any problems like this? Any advice or thought?

  232. dennisb - Auto Tool Sales :

    Date: March 7, 2014 @ 11:05 am

    Heather, not sure what what year make or model you have but lean codes are usually caused from vacuum leaks especially on Ford vehicles. Checking vacuum leaks with spray used to work great on non computerized vehicles. But checking especially smaller vacuum leaks on computerized vehicles is harder and usually requires checking the trim data with a scan tool. A smoke test is a good way to test for vacuum leaks. It should be checked with the engine cold and also at operating temperature due to heat and cold sometimes sealing certain leaks temporarily. Once the repair has been made and the P0171 and 74 codes don’t come back, see if the other issues are present. The other running issues may be resolved too. If not they can be diagnosed at that time. Good Luck.

  233. Heather :

    Date: March 7, 2014 @ 11:12 am

    It’s a 2000 Ford Explorer

  234. Joe Pa P :

    Date: March 12, 2014 @ 5:21 pm

    I just found this site. I applaud you for providing such a service. Thank YOU!

    We have a 2006 Malibu V6 61,000 miles that is having the same issues as most of problems listed (codes Po171 po174 and P1516). First a little history. I’m not sure the codes are related to a problem that developed a week prior to the service engine soon light coming on. A week prior – When accelerating on a hill, the wheels spun on ice and the TC (traction control) light came on and stayed on. After checking on-line I found that if the car was moved forward then back three times the TC would reset. I did as suggested and the TC issue was resolved (but only until the next problem surfaced). Then, a week later, while stopped at a traffic light the car was rear ended. Other than the some damage to the trunk and bumper the car could be driven. However, less than a mile after driving away the service engine soon light came on. When the repairs were made and the codes checked they found the PO171 PO174 and PO1516. The insurance adjuster said he doubted the accident caused the codes. He did however say, if it could be proved that the accident caused the codes, the insurance company would pay the repair bill. I asked the body shop if they could do a smoke test. They could not, but said they sent the car to a repair shop to have the test done. They were told everything checked out ok and the codes were cleared. I’m thinking the body shop may have cleared the codes so I would pay them, because a week after getting the car back the service engine soon light is back on. This is not the car I usually drive, so I could only relay what my wife told me happened. That was until I took the car for a drive and experienced the problem. When the light comes on the Reduce Power message scrolls across the message center and the car feels like someone just pulled you from behind, the car feels like it wants to stall, which my wife said did happen to her. I should add that the service engine soon light has been an intermittent problem since the accident. And. The auto starter will not work if the car was turned off when the service engine soon light was on.

    I had also asked the body shop manager if there were any vacuum hoses in the rear of the car and was told there were not.

    Please give us some direction, as we are going to bring the car to a Chevy service center and before we start replacing parts at the discretion of the dealer.

  235. dennisb - Auto Tool Sales :

    Date: March 12, 2014 @ 5:40 pm

    Hello Joe, thanks for visiting. The P1516 code for the Chevrolet Malibu means “Throttle Actuator Control Module Throttle Actuator Position Performance”. Which could mean there’s a fault in the throttle control module, wiring or software issue. Not likely that this was caused by an accident or the traction control. But it could cause the vehicle to slow down as you’ve described. The Chevy dealer will undoubtedly know what the common failure is.

    The P0171 and P0174 codes are lean codes which are usually caused by a vacuum leak. Vacuum leaks aren’t usually a big issue with Chevy, but anything is possible. An exhaust leak before any of the Oxygen sensors could also cause these codes. Again not likely that this was caused by an accident or the traction control.

    The accident could have conceivably caused EVAP codes, having to do with the containment and handling of fuel vapors, but you have no related codes for the Evaporative system. I say that because there’s the vapor lines going to and from the fuel tank. If there are no Evap codes then those lines should be fine. Good Luck.

  236. Joe Pa P :

    Date: March 13, 2014 @ 11:35 am

    Wow! This is great. And, such a quick response. What a wonderful service. Thank you so much for the feed back……Joe

  237. usmcdemoss :

    Date: March 14, 2014 @ 2:42 pm

    I have a 1998 F150 4.6l. Check engine light came on and it says that its too lean on banks 1 and 2 (P0171 P0174.)The truck seems to be running fine except when comming to a stop, the truck will shut off. I was checking for vacume leaks today, I couldnt hear anything while the truck was running, but when i shut it off i could hear a hissing sound. I check the 2 rubber elbows (1 up next to the throttle body and the other that hooks to the same plastic hose at the bottom.) They looked a little dry rotted where they connect but not on the actual elbow, but it didnt look bad enough to be the cause. I taped them up to see if that was the problem. It seemed to help a little, but not much. I can still hear the hissing sound when i shut the truck off, but i can locate it. Are there anymore of those rubber elbows? Because 2 is all i seen. I also checked the MAF sensor and it looked fine. I believe there is a exhaust leak comming out of the rear of the manifolds, could that make the truck stall and throw those lean codes?

  238. usmcdemoss :

    Date: March 14, 2014 @ 2:53 pm

    I also just replace all 8 plugs because i was have misfires on cylinders 1-4. The fuel line connector where it connects the the fuel rail and the injectors look rusty, and may need replaced, but i dont want to just start throwing money out and it not be the problem. I’d rather fix 1 problem at a time.

  239. dennisb - Auto Tool Sales :

    Date: March 14, 2014 @ 3:00 pm

    The exhaust leaks are letting in air that can definitely cause lean codes. Since tape helped “some” on the hoses I would replace them and recheck after doing all the repairs, clearing the codes and driving for long enough for the computer to do a self test. This may take a 20 or 30 miles depending upon the conditions driven. The hissing sound when you turn it off could be normal, just air going into the intake and abruptly stopping when the engine is turned off. I would also look for any other hoses coming off the intake that may have leaks. Looking at the MAF (Mass Air Flow) is good as far as a visual inspection but they can look good and be faulty. The good news is you probably found the main problems with the hoses and the exhaust leak. Good Luck.

  240. dennisb - Auto Tool Sales :

    Date: March 14, 2014 @ 3:02 pm

    Misfires are unrelated to lean codes in most cases. The rusty fuel line connector could be a safety issue if it doesn’t hold. Good Luck.

  241. usmcdemoss :

    Date: March 14, 2014 @ 4:07 pm

    ok, thanks for gettin back with me so quick. I did find 1 more hose that hooks to the back of the throttle body and runs to the clutch fluid resevoir that looks dry rotted. Im going to replace that hose and the 2 elbows and see if i can take care of the exhaust leaks myself or take it somewhere. Thanks for the help.

  242. usmcdemoss :

    Date: March 14, 2014 @ 6:21 pm

    I was going to get the hose and rubber elbow, I stopped at my inlaws and noticed antifreeze pouring out the bottom, I popped the hood and its all over the passenger side of motor and battery. Im hoping none got down on my plugs. I just cant win for losing lol. But I believe I found the main problem and its that blue oval on the front of the truck lol

  243. dennisb - Auto Tool Sales :

    Date: March 15, 2014 @ 8:31 am

    Ouch that cuts deep into the heart of all of us Ford lover’s ;).

  244. Heather :

    Date: March 27, 2014 @ 5:57 pm

    My Explorer is a 2000 4.0 V6 SOHC E engine.

    Another shop told me I needed new Upper and Lower intake manifold gaskets after my explorer failed Emissions testing and they did a diagnostics. Well, I changed the gaskets and I still have the same 2 codes P0171 and P0174 running lean. Does it take some time for the car to realized that I fixed the problem and to stop running lean? Or what? The shop said this is what the problem was but they only guaranteed it if they did it themselves. So if that didn’t fix the problem they would have ate the cost fixing other things till it fixed the engine light. But for the price they were going to charge me for it, I did it myself. They wanted to charge me $541.38 for the parts, which was for the upper and lower gaskets and the EGR o-ring. That is insane. I got the upper and lower gaskets from AutoZone for $16.99 and the O-ring for $1.99. They marked the prices up on that crazy crazy high. And their labor cost was $308.67, which seems a bit high also.

    So like I said I change the gaskets and o-ring like they said but it still didn’t clear the code. They said they could do a computer update to set it back to normal, but they couldn’t guarantee it would fix the codes, for $96. And they could do the diagnostics AGAIN for another $96. I don’t want to keep throwing money that I don’t have into this car if they aren’t going to give me the right answer. I’m not sure what to do at this point. Any advice? Do you think the codes will reset after a while of driving? Do know how long? I only have a few days to get it to pass inspection before my plate tags are up.

  245. dennisb - Auto Tool Sales :

    Date: March 28, 2014 @ 7:19 am

    Heather, the codes have to be cleared with a code reader or scan tool. See the following link for an affordable code reader that will pay for itself very quickly with only a few uses – clear codes with this code reader. If the codes are not cleared it will take several trips (as defined by Ford) for the CEL (Check Engine Light) to go off by itself. This is because the computer will run self tests at different times and driving speeds etc, to ensure the vehicle’s systems pass many tests. All readings must be within certain parameters which tells the computer everything is ok. But even if you clear the codes, if the repair was not done properly (like surfaces not cleaned well or a folded gasket, a cut seal etc) or if there’s another cause the CEL will come back on. That’s because the computer is always looking for failed tests. Good Luck.

  246. Heather :

    Date: March 28, 2014 @ 10:40 am

    I have a scan tool and I have cleared the codes like 8 times since I have changed the gaskets, in a day and a half ago. It probably should have corrected itself by then? I don’t know. I don’t know what to do next.

  247. dennisb - Auto Tool Sales :

    Date: March 28, 2014 @ 11:17 am

    Heather, it’s not a matter of it correcting itself. If you cleared the codes and they keep coming back there’s still a problem. Recheck your work and look for any other vacuum leaks you may have created by leaving something loose like a vacuum line or intake hose etc.

    Among other repair shops, I used to work at Pep Boys. Pep Boys has a lot of do it yourselfers that wanted us to diagnose problems so they could walk into the store and buy parts so they could fix the car themselves. Many times though, customers would bring the car back to us to recheck after they did the work. Most of the time they caused more problems than what they had to begin with. Tools and parts still require an experienced technician that know what to do with them. I’m not saying that you are not mechanically inclined but there’s no substitute for experience. Good Luck.

  248. Heather :

    Date: March 28, 2014 @ 1:58 pm

    I understand that. I didn’t actually do it myself, my mothers husband did it. He was the top mechanic at a major car facility, now retired. I guess I will have to take it in and pay for more diagnostics. :(

  249. Usmcdemoss :

    Date: April 1, 2014 @ 3:22 pm

    Fixed exhaust leak, seemed to help a lot. CEL went off but came back on. Which I expected because the exhaust still leaks a little bit right above the 02 sensor. I went to auto zone to scan it to make sure and b4 I left I crawled up underneath and notice that it’s pouring cooling. It’s draining down on the exhaust manifold on passenger side. Think it’s a head gasket?

  250. dennisb - Auto Tool Sales :

    Date: April 1, 2014 @ 4:58 pm

    I would pressure test the cooling system to find the leak. Good Luck.

  251. redjep :

    Date: April 20, 2014 @ 5:58 am

    I have a Nissan AD wagon 2006 1.8. The Check Engine Light came on and the code is 0171. It came on about a month ago. There is no vacuum leak, changed fuel injectors, air filter, spark plugs and 02 sensor light went off. The vehicle drives good no Check Engine Light until after a week, the light came back on. If the head gasket was changed and the cylinder head was shaved can that cause a problem with the reading? I dont know what to change now.

  252. dennisb - Auto Tool Sales :

    Date: April 20, 2014 @ 8:09 am

    The cylinder head being machined when the head gasket was changed will not cause a P0171 lean code. The most likely cause of most lean codes are vacuum leaks, exhaust leaks before the 02 sensors (which Nissan calls AF (Air Fuel) Sensors or a cold air leak between the MAF (Mass Air Flow) Sensor and the throttle body. The lean code means there’s more Oxygen being detected in the exhaust than what the computer expects to see. It can end up there by the in several ways; Vacuum Leak, Exhaust Leak or Cold Air Intake leak. On some VW’s and Nissans a faulty MAF can also fool the computer into thinking there’s too much Oxygen in the exhaust too. But I would check the top three causes first. 1. Vacuum Leaks 2. Exhaust Leak 3. Cold Air intake leak. Good Luck.

  253. J. Locu :

    Date: April 29, 2014 @ 4:50 am

    Also have lean codes P0171 and P0174, started few days after passing year 2012 emission test in Washington State. 1997 Ford Ranger, 4X4, 6 cyl, 4.0 liter. A year and a half ago I paid a popular national tire/car care center diagnosed troubles – fuel injectors issue, a $800+; took it for second opinion, paid a regionally known repair shop – dismissed injectors, instead called it upper and lower intake manifold gaskets leak, similar $800+ job. Took a third opinion from an independent repair shop; dismissed both injectors and manifold gaskets issues, and stated about $480 new fuel pump job.

    Took several free scans from auto parts stores and all came up with same fault codes P0171 and P0174 saying all of the above is posible.

    Took truck for the 4th time to another repair shop explaining all of the above. Mechanic there told me to first go take the emission test anyway and then get back to him with the emission test results then he will use the emission test result as a basis to fix it.

    So I failed emission test 3 months ago then took vehicle back to the mechanic a month ago when I got the $350 he wanted. Mechanic replaced two oxygen sensors and erased the two fault codes then told me to go get the emission retest; two fault codes came back on before arriving for the emission retest but I got a ‘passed test’ because the mechanic is a certified emission specialist and I had spent more than the $150+ exemption. I told the mechanic that the same fault codes came back on. This time the mechanic is blaming the vehicle computer although it is the same trouble codes he tried to fix.

    After searching Youtube yesterday on how to fix P0171 and P0174 I made a simple vacuum leak smoke tester. I found that there are two major smoke leaks. To pump air inside vehicle intake manifold through the home made smoke device, I used an old Black and Decker 18 volts battery cordless leaf blower, with a battery that was about two/third discharged and found the Throttle Position Sensor (TPS) leaks smoke noticeably heavy.

    Interestingly, whenever I used a battery that is only about half way discharged, the ERG valve also leaks smoke leaking as much smoke as the TPS. Going back with the first battery, only the TPS showed smoke leak.

    Is there a known psi setting when pumping air/smoke into the intake when using the smoke test?

    I have never heard of a vacuum leak on the Throttle Position Sensor before. I will do more testing tomorrow, will use silicon paste to seal around the mounting areas, as is, of the parts that are leaking, will let dry the seal then try with the smoke device again. If the silicon seal stops vacuum leak i will either leave the silicon seal as permanent seal or replace gaskets. Otherwise I will completely replace each of the parts that are leaking smoke.

    I will also go back to the mechanic and demand replacement parts for the the two oxygen sensors because exactly the same trouble codes reappeared within 30 minutes or after driving 16 miles since he replaced the two sensors. I will not have him install the parts.

    It feel like I’m going to run out of mechanics.

    Two years ago I took a 1994 Nissan Quest, 150,000 mileage, to an independent shop, a transmission shop, and two other repair shops because it was idling/ running/ sounding terrible. After each repair shops and totaling close to $2000, not much improvement. What I found out after playing around with the throttle position adjustment was that only one of the four engine mounts is in good shape. A surprise because had them replaced within one year period by one of the shop mentioned here; it either didn’t do a good job or maybe only replaced one mount and charged me for four mounts. On these vans, the idle/throttle/cruise control cable location in the engine compartment, adjustment is influenced heavily by the status of the engine mounts. I consider it a bad design. The van/engine computer is less able to fine tune the adjustment the more the engine mounts sag. A particular combination of bad and good mounts actually makes it worst.

  254. dennisb - Auto Tool Sales :

    Date: April 29, 2014 @ 7:39 am

    First, the most common cause of lean codes are vacuum leaks. The most common cause of vacuum leaks on the Ford 4.0 is upper and and lower intake seals. I’ve replaced many of them and have successfully fixed the problem. They are hard to check even with a real smoke machine. BTW about 1 psi is the correct pressure. Anything more and you could potentially cause leaks. The EGR will naturally leak when doing a smoke test. If you really think it’s bad I would vacuum test with a hand pumped vacuum gauge to see if the diaphragm leaks or not. For the low cost of the intake seals, I would replace them and clear the codes. Chances are very good that this will fix the problem since they are a well known common failure pattern on the Ford 4.0. Good Luck.

  255. Jesse :

    Date: May 14, 2014 @ 1:37 am

    5-14-14,I have an 2004 Suzuki Verona Engine & ABS lights on,with these codes: P1382 Manufacture,P0171 bank 1system 2 leak, P0174 bank 2 system 2, P0133 original bank 1 sensor 1, P0304 cylinder 4 misfires,P0121 throttle position sensor,& 0102.Car skips, runs rough especially when cold,& a sound is coming from behind the engine when standing out front of car. I need your advise.

  256. dennisb - Auto Tool Sales :

    Date: May 14, 2014 @ 7:57 am

    Jesse, that’s a lot of codes. It’s much easier to diagnose engine codes/problems individually when they first start. I imagine this vehicle has been driven a long time with the CEL on and codes kept setting but the light was already on from the first one set. Once the vehicle reached a point when it was not hardly driva-able a solution was searched. First the ABS has nothing to do with the “P” codes. The “P” stands for Powertrain. The ABS will need to be diagnosed separately. There’s not a lot of information on the P1382. In fact some websites are incorrectly identifying this as an ABS code. ABS codes do not start with a P because P stands for Powertrain. You may need to acquire a service manual for Suzuki to get more info on the P1382 which is most likely related to the Cam or distributor timing (if applicable). As far as the other codes, the most serious ones are usaully listed first. You already know the P0171 and P0174 usually mean an engine vacuum leak or exhaust leak before the second sensors. The P0133 Oxygen sensor code could be legitamite or caused by one of the other issues. The P0304 cylinder misfire could be from a bad spark plug or coil. It could also be from something more serious like bad compression from a ring or valve. The P0121 usually is from a bad throttle position sensor or it’s wiring. The sound that you are hearing; I could only guess that it is from a leaky exhaust manifold or from a misfire causing a slight backfire. This may be a good candidate for a local technician to diagnose since there are probably several issues at play here. Good Luck.

  257. Jessica Sessions :

    Date: May 17, 2014 @ 6:36 pm

    Hi Dennis we have a 2003 ford Taurus and the engine light is on it hisses while both idling and braking and it idles hard not sure what to do we replaced the egr sensor amongst other things

  258. dennisb - Auto Tool Sales :

    Date: May 17, 2014 @ 8:03 pm

    I would check the codes and go from there. If you get lean codes and hear hissing inside the vehicle especially when braking you may have a brake power booster leaking vacuum. Good Luck.

  259. Kevin :

    Date: May 26, 2014 @ 4:30 pm

    Dennis, I could really use your help. 2003 Ford Explorer, 4.0L V6. P0171 and 0174 codes. Had it to two different mechanics. The intake manifold gaskets were changed by both mechanics (the second one said the first one didn’t clean the manifold enough), fuel rail pressures ok, cleaned fuel injectors, new fuel injector O rings, new MAF and new upstream O2 sensors. PCV tube and elbow are good (I took them off and pressure tested them). I used propane to help locate a vacuum leak–nothing after about 2 cans worth (watching fuel trims on OBD2). The left bank shows higher STFT % than the right, but after driving for a while they both max out the LTFT at 25%. I voltage tested the TPS and it goes from 0.86-4.53 evenly. Mileage about 120K. I also replaced the manifold isolation bolts with the updated ones and torqued to 89 in pounds.
    The second mechanic said it must be clogged catalytic convertors, but I know you have said that that is Never the cause of 0171 and 0174.
    We purchased the car used last year, and I don’t know if the PCM has been updated.
    What do you suggest at this point? I’m running out of patience and $!

  260. dennisb - Auto Tool Sales :

    Date: May 26, 2014 @ 4:48 pm

    Kevin, vacuum leaks are hard to locate sometimes. Lower intake gaskets are the most common cause of lean codes on the Explorer (wasn’t sure if your mechanics changed the easier upper ones only). Catalytic converters shouldn’t cause lean codes unless they leak exhaust. I would have a shop smoke test it (cold and warm) and go from there. Good Luck.

  261. Kevin :

    Date: May 26, 2014 @ 6:02 pm

    he changed the ones between the intake manifold (plastic) and the metal intake on the engine. There are 6 oval gaskets. I thought there were only these 6 gaskets on the v6 4.0L model? Interesting observation—-after the motor has been running, when I turn the motor off, after about 5 seconds, there is this noise, best described as a “farting noise”, which seems to come from below the intake manifold about the level of the middle cylinders, maybe back a little bit. Not sure if that is related. I will have them perform the smoke test again and see what is up. Maybe it was leaking and they installed some inferior aftermarket gaskets, or didn’t get them to seat properly. What do you think about he PCM update? Would that make any difference?

  262. Kevin :

    Date: May 26, 2014 @ 6:49 pm

    I think on the 2003 model the intake manifold is one piece, and there are only the two rows of three gaskets. I think on an earlier model there was some intermediate piece which had some gaskets that looked like the number “8″, and them some oval gaskets

  263. Jayden :

    Date: June 13, 2014 @ 12:16 am

    Hi Dennis, I have a 2004 Suzuki Verona with the following codes: P0171, P0174, and P1670. Any suggestions? Thanks in advance!

  264. dennisb - Auto Tool Sales :

    Date: June 13, 2014 @ 6:36 am

    Check for vacuum and exhaust leaks. The P1670 seems to be a communication error. I haven’t dealt with that code before. Good Luck.

  265. Jayden :

    Date: June 13, 2014 @ 10:23 am

    Thanks much for your feedback, Dennis. I’m not sure either about that P1670 code. I was told by a local mechanic that the P071, P074, and the P1670 were all related, or had to do with the same problem… its just confusing. I do however, wanted to ask you about another issue with my Suzuki Verona. During the day time, the DRL lights stay on, but once it gets really dark, the lights goes off. If it wasn’t because of my fog lights, I would be completely in the dark at night. Would that be a fuse or relay problem? Or Electrical? Thanks for your help…

  266. dennisb - Auto Tool Sales :

    Date: June 13, 2014 @ 10:53 am

    Those codes may be related or they might not. The P0171 and P0174 are lean codes which are usually related to vacuum leaks or an exhaust leak which results in the Oxygen sensors detecting more oxygen in the exhaust than expected. The P1670 is a communication error (BUS error) which may not have anything to do with the other faults. I’ve heard of a few problems with the low beams and day time running lights not working properly but not the exact problem you are experiencing. I would get a wiring diagram and check the circuit. It takes experience testing electrical circuits and the ability to read wiring diagrams. Good Luck.

  267. Harry :

    Date: June 17, 2014 @ 3:32 am

    I have an 04 town car and after my vehicle was in an auto accident (side swiped on the driver’s side by a driver not paying attention) my car started stalling. I took my vehicle to the auto store and received both 174 & 171 codes. On my way home from the auto store I could hear the hissing noise and immediately parked my car; I didn’t want to further the problem. So after a little research and rereading the previous reviews I’ve come to the conclusion the impact from the accident caused the vacuum house to malfunction. Was fixed at the dealership at no cost to me.

  268. dennisb - Auto Tool Sales :

    Date: June 17, 2014 @ 8:29 am

    Great. Glad the article helped.

  269. Jayden :

    Date: June 18, 2014 @ 3:51 am

    Hi Dennis, I’m back with more questions and concerns for you. Only the driver window is barely working, while the other 3 power windows are not functioning properly. Also, I put a bottle of Freon on my car and the a/c was really cold, but it only lasted a day or so. Would that be a compressor problem, or broken hoses somewhere? Finally, if you have the time I would like to ask your opinion on why my car clock gives different time readings. For instance, when I start the car, the clock usually displays the correct time. Then it shuts off completely for a few minutes and turns back on with a different time display after. Thanks

    Jayden

  270. dennisb - Auto Tool Sales :

    Date: June 18, 2014 @ 7:39 am

    Obviously there’s a refrigerant leak. The leak does not have to be from the compresssor, although it could be. Any part of the system that refrigerant circulates through could be the cause of the leak. A knowledgeable technician with a good refrigerant leak detector can pinpoint the leak and provide an estimate for repair. Read more about diagnosing car Freon Leaks in our other AC articles.

    As far as the clock; Clocks used to be all mechanical, so they could be taken apart and repaired. With todays electronic clocks, when something goes bad it’s usually less expensive to replace it rather than repair it. The exception may be if it’s only serviceable with other components. Part of a larger assembly, like the instrument cluster for example. In that case the assembly can be taken out and sent to a repair specialist. They can fix it and send it back for re-installation. Good Luck.

  271. Jayden :

    Date: June 21, 2014 @ 1:51 pm

    Hi Dennis,

    I took my car to a repair shop and they replaced the intake gasket and seals but the same codes came back again just after 2 days. I reset it the codes and took it for inspection (2nd time), but the inspection technician couldn’t proceed because 3 things were still not ready. The (OBD) monitor results were as follow: Catalyst Monitor- Not Ready; Evaporative System Monitor- Not Ready; 02 Sensor Monitor- Not Ready. The technician suggested to run the car for a 100 miles and bring it back. Do you have any other suggestions besides that? Thanks

    Jayden

  272. dennisb - Auto Tool Sales :

    Date: June 21, 2014 @ 2:33 pm

    No need to wait, to do a smoke test. The inspection tech see’s that you just cleared the codes. That’s why he wants you to run it a while before bringing it back. This will give all the self checks time to run. Good Luck.

  273. Jayden :

    Date: July 2, 2014 @ 12:12 pm

    Hello Dennis,

    I just came back with another failed inspection, this time because its (OBD) could not communicate with the test equipment. Everything else passed. Do you think that’s why I’m getting that communication error (P1670) code? Any insight would help a lot. Thanks

    Jayden

  274. dennisb - Auto Tool Sales :

    Date: July 2, 2014 @ 1:15 pm

    Probably not related. If there was communication through the OBD II connector is the only way to get a code to begin with. Sometimes the OBD II connector harness shares power (uses same fuse) with the auxiliary power outlet. Sometimes it’s on a different circuit. In either case, there should be power on # 16 pin. If there’s not, start checking all your fuses. Sometimes there are multiple fuse panel locations, so check your owners manual if needed.

    OBD II Connector

  275. Jayden :

    Date: July 2, 2014 @ 2:00 pm

    Awesome, thank you!

  276. Alex :

    Date: July 22, 2014 @ 11:25 pm

    Dennis

    I have an 01f150 5.4L when the truck is cold before reaching temperature it runs fine as soon as truck reaches temp. It loses serious power. I changed the mass air flow sensor and checked the fuel pressure and it is good. Truck does not have cats at all and hasn’t for 2 years. Problem started two months ago. It also throws the bank 1and2 system to lean codes. Any ideas? Please help! The truck also has a k&n cold air intake.

  277. dennisb - Auto Tool Sales :

    Date: July 23, 2014 @ 7:34 am

    Alex, the lean codes would have started right after the catalytic converters were removed 2 years ago. The rear Oxygen sensors are seeing more Oxygen because it is not being used to burn excess fuel in the cats as it is designed to do. Your computer in the truck is trying to compensate for skewed readings (high Oxygen/Lean, readings). The computer has no way of knowing the cat’s were removed and is adjusting fuel/air mixture to try to get the readings back to where they should be. The result is too much fuel being dumped into the system. It’s no wonder that performance has been affected. The only puzzling thing is that problems only started recently and not when the cats were first removed. Perhaps the mufflers are restricted now from all the extra unburned fuel. Also, there are huge fines if caught with no cats, for tampering with emission control devices.

  278. Denlors Auto Blog » Blog Archive Ford P1131, P1151 Fault Codes 3.0 DOHC – O-2 Diagnostics | Denlors Auto Blog :

    Date: July 23, 2014 @ 12:34 pm

    […] P0171, P0174 codes or both have been pulled using a scan tool see our related automotive repair blog for more free tips diagnosing lean codes. Question – I have a Ford Taurus and want to […]

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