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.

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

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?

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

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

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.

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.

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

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?

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?

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.

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.

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

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.

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 ????

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

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?

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!

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

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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?

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!!!

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.

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.

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???

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…

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.

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…..

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!

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.

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

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!!!

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?

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.

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.

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.

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.

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

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?

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.

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.

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?

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!

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.

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

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.

have any ideas what else it could be?

ive read about 4 elbows on the pvc line also.

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.

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.

“the most likely cause is a vacuum leak”

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.

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.

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

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.

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?

’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…

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?

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?

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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!!!