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Car Heater Blows Cold – Auto Service Tips Part 2

11:23 am Consumer, DIY, How To Auto Repair

The standard heater core above is copper - some cores are aluminum .

© DenLorsTools.com Summary: This is part two of our exclusive car repair blog helping to troubleshoot car heater problems when they blow cold. The original article has been very popular in the colder months of the year. This repair article picks up where the first article leaves off. If you have not read the first part of the Car Heater Blows Cold article, I recommend reading it first.

One common complaint with the car heater performance; is the temperature of the air only blows warm for a few seconds and then starts to blow cold. This could be caused by a restricted heater core or a thermostat that is staying open – or the t-stat is opening too soon.

Clogged Heater Core


If the heater core is clogged up, the coolant in it may be hot initially. However, after the air blows through the heater core it lowers the temperature of the coolant inside of it. If the flow is restricted it will cool the coolant down to a point that results in the air blowing through the fins to NOT be heated as designed. Low coolant flow within the core can sometimes be deceiving because both heater hoses may feel hot to the touch. As talked about in part one of this article, one test is to check the temp of both hoses to see if they are both hot to the touch. If the core is completely stopped up, one hose will be cold. The possible curve ball to this test is that there may be enough flow to make the hoses feel hot to the touch, but not enough flow to maintain a high enough temperature when the heater is being operated. This may result in warm air blowing through the vents only for a few seconds. The easiest way to check for volume of flow is by removing both heater hoses and spraying water through the core with a garden hose. If only a trickle comes out, then a restricted core is confirmed. There should be free flowing water exiting the core when this is being done. Just as much water coming out as being sprayed in. Sometimes the heater core can be flushed to increase flow and other times the core has to be replaced.

Malfunctioning Thermostat

If the thermostat stays open all the time, or opens too early (before the engine reaches the target operating temperature) the heater will not blow as warm as it should. If this is the case, usually after the car has ran awhile the vent temperature will increase gradually (at varying rates depending on the ambient temp). If it’s really cold outside the engine’s temperature gauge may stay on cold. If the engine’s temp never reaches 1/2 way or the normal range, the heater cannot be expected to heat up either since it operates by exchanging heat from the engine’s coolant. This is why warm air may blow for a few seconds when first turned on, but after the core is cooled off, cold air will come out of the vents. Also, many times if the thermostat is not closing correctly, a CEL (Check Engine Light) will be turned on with a fault code stored. The code that can be set is a P1281, see the articles below for tips on dealing with this code on the Jeep Grand Cherokee and the Chevrolet Trailblazer.

Related Thermostat Auto Repair Articles

P1281 Code GMC Chevy Thermostat – Scanner Code Reader

Jeep Grand Cherokee Scanned P1281 Fault Code

Heater blows warm a little when RPMs are raised?

Rusty Car Water Pump Impeller

A common occurrence is the heat blows warm or “warmer” when the gas pedal is pressed slightly. When the engine is turning faster (higher RPMs) the water pump turns faster too. When the waterpump is pumping water faster there’s more water volume traveling through the heater core. A little increase in temp is normal. Extreme differences are felt when the system isn’t working as it is suppose to. As mentioned in part one of this article, air trapped in the cooling system can affect the heater performance. Bleeding the air out is the first thing to try in this case. What if the air is bled out, the heater core is not restricted, the thermostat is new, there’s not an actuator or door issue and there’s not much heat blowing out the vents? Next thing to investigate is the water pump. Sometimes though it is seldom; the water pump may not be pumping sufficiently. The water pump impeller may have rusted fins on the impeller. A special thanks to workshop.search-autoparts.com for providing the image of the rusty water pump. In the summer this pump would cause an overheat – in the winter it would cause the heater performance to suffer. One way to check for a problem like this is to do the following.

Measure three temperature readings from the car with an infrared thermometer.

1. Radiator temperature. 2. Engine block. 3. The vent temp with the heater on maximum.

The three temperature readings could indicate that the water is not flowing properly. A cold vent temp with a “warm” engine and a cool reading at the radiator is the main indicator.

A more direct way to check flow from the water pump is to remove the upper radiator hose from the radiator. Have someone start the engine for a few seconds to see if there is water pumping out.

The impeller on the water pump could come loose or can deteriorate from rust or electrolysis due to lack of maintenance or improper ground straps. Still would like more tips? See Car Heater Not Working? Troubleshooting, How to Fix Repair Tips.

Automotive Heater that plugs into car cigarette lighter – Review

What about the car heater that plugs into the cigarette lighter? There are several brands of these available on the Internet that provide the hope of a cheap fix for a malfunctioning car heater. Nice thought, but just like most things in life that sounds too good to be true it usually is, this one is too good to be true also. The heating element is very small and inadequate for making a real difference in the passenger compartment temperature. The fan doesn’t put much volume out and if it did, the air would not heat up even as much as it does. There are several models that have a slightly larger size, the problem is with those units they usually end up blowing a fuse. Larger fuse anyone? Not a good idea, this might melt wiring in the car and could likely be a fire hazard. There have even been some reports of these units melting! Save yourself the trouble and expense and work towards fixing the heater system your car came with.

Click the following for even more on car heater not working, with tips for troubleshooting problems.

 

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30 Responses
  1. Devin :

    Date: January 4, 2011 @ 12:37 pm

    I have a 2000 Chevy S10. After a few minutes from starting it up, I start to get warm air from the heater, but only for about 5-10 minutes, then it goes cold. I’ve also noticed the truck’s temperature only goes up to and stays around the quarter mark on the gauge. Which would be around 150-155 degrees. Does that sound like a bad thermostat?

  2. dennisb - Auto Tool Sales :

    Date: January 4, 2011 @ 12:55 pm

    Devin, It could be a bad thermostat. It could be opening too soon or the spring could be out of position. I would raise the RPM’s to 2,000 to see if the temp increases and if the heater starts to work. This is a simple thing to try without taking anything apart. Good Luck.

  3. Devin :

    Date: January 5, 2011 @ 10:26 am

    Raising the RPM’s didn’t seem to work at all.

    .

  4. dennisb :

    Date: January 6, 2011 @ 9:32 am

    I would check for flow and a possibly blocked heater core. Sometimes there’s no substititution for getting your hands a little dirty and manually checking things out. If you need further guidance, you may want to ask your question to a dealer tech by using the question box from Do-it-Yourself Car Repair (Expert Car Advice) – just ask and get quick answers! Good Luck.

  5. dennisb - Auto Tool Sales :

    Date: January 7, 2011 @ 9:49 am

    Click the following for a little about my work backgound:

    About

  6. jon jackson :

    Date: January 7, 2011 @ 9:51 am

    i was told by local dealer as you suggested and they told me to check circuit 35. what is this circuit 35 and where can it be found.

    jon
    2004 trailblazer with p1281 code

  7. dennisb - Auto Tool Sales :

    Date: January 7, 2011 @ 10:09 am

    Jon, circuit 35 is an electrical circuit, see a repair manual if you think your problem is an electrical one. However, I would first address the P1281 fault code which has to do with the t-stat not fuctioning properly. See our related article, click below. If you still need guidance I recommend asking a dealer tech by using the question box – not necessarily visiting the dealer.

    Trailblazer P1281

  8. J West :

    Date: January 12, 2011 @ 2:41 am

    On Pontiacs there is always the alternative that an internal gasket leak has begun and you will see this first as a decrease in heater temperature with an increase only at high RPM’s, then later you will get the low coolant light continuously coming on without any visual external leaks or noticeable decrease in fluid and then next you will notice fluid loss still without external leaks and then it will progress to the overflowing reservoir that does not return fluid to the radiator and then overheating issues all starting from the first symptom of the heater blowing cold air at low RPM’s and then finally you will see that antifreeze has made it into the oil pan by seeing the milky substance on your dip stick. Before this point you may have already flushed the cooling system, replaced the radiator cap and the thermostat, bled the system of air, confirmed that the heater core and water pump were working properly and checked for compression loss only to have led to an expensive repair stemming from the upper and then lower manifold gaskets or (Plenum) These engine’s upper intake plenum is prone to getting an internal crack in the composite plastic. If these symptoms sound familiar get it fixed immediately don’t mess with replacing other parts it is always the same problem with Pontiac Montana’s 95 to 2005.

  9. Evan :

    Date: September 21, 2013 @ 1:33 pm

    I have a 1983 Lincoln and it doesnt blow out heat or air from the vents just cold air from the defrost.My coolant level is fine.Thank-you.

  10. dennisb - Auto Tool Sales :

    Date: September 21, 2013 @ 3:51 pm

    Only blowing out the defrost is a control issue. Maybe a vacuum leak. If it’s not blowing out the defrost hot, that could be an air lock or a clogged core. On a vehicle that old, a 1983 (no offense) I wouldn’t be surprised if someone has by-passed the heater core at some point due to a leak and the expense and labor involved in replacing it. Good Luck.

  11. Christy :

    Date: November 4, 2013 @ 11:44 am

    I have a 2000 pontiac grand am. My car is not pumping out hot air, only cold. I just recently had the water pump and hoses replaced due to a leak, but this did not change the heating issue. My thermostat will randomly go sky high and send me a warning, usually this happens with in about 10 mins of starting the car and driving. If I put the heater on, then the thermostat goes back to normal, but it still continuously pumps out cold air. What could this possibly be? Any help is greatly appreciated!

  12. dennisb - Auto Tool Sales :

    Date: November 4, 2013 @ 12:11 pm

    Christy, the gauge is what displays the temperature not the thermostat. It’s likely that there’s a lot of air in the engine’s cooling system. Once the system is bled and there’s no air lock the heater should start working and the engine will run at the correct temperature. Some Pontiac Grand Ams have a bleeder screw on a coolant pipe used to bleed the air out of the coolant system. Have your technician look for that and refer to a repair manual if needed. If the air cannot be bled out, he may want to check for a blown head gasket by doing a carbon test to see if there’s exhaust in the cooling system. Good Luck.

  13. Amy :

    Date: November 12, 2013 @ 7:00 am

    Hi, I have a 2002 Jeep Liberty Limited and its been blowing cold air for a year now. We’ve done all the “cheap fixes” like new thermostat and blowing out the line. The mechanic said its either the blend motor or heating core. The thing is the passenger side on the bottom blows warm air. I’m hoping its the blend motor and my husband would rather do it himself to save us money. Is it possible to repair ourselves? My husband will bring it in to a mechanic if it is the heating core if the blend motor is working as it should.
    Thank you for reading and hopefully answering my question.

  14. dennisb - Auto Tool Sales :

    Date: November 12, 2013 @ 7:25 am

    Amy, I would take it to a different mechanic. The reason is because your present mechanic says it’s “either a blend motor or the the heater core“. That tells me he has no clue because those are two completely different things. This sounds like the guess of an auto repair blogger, thousands of miles away, that doesn’t have the benefit of checking the vehicle personally.

    If both, the inlet and outlet heater hoses are hot, that would indicate that the heater core is not complete clogged. The fact that you are getting warm air from any vent also supports that. It’s easy to spray a garden hose through the heater core to check the flow with the heater hoses off, if you suspect the core is clogged partially. If all the modes work (you can change which vents the air comes out) the doors and actuators/motors should be OK. The blend door actuator controls the mix of cold and hot air; I would look to see if I could access the blend actuator to see if it is working. If it can be accessed without removing the dash, it could be removed and the door worked manually to test, to see if hot air can be managed. As far as your husband doing the work; Not sure. I’ve had Liberty dashes out plenty of times to replace A/C evaporator cores. But I don’t remember the difficulty level of just changing an actuator for the blend function. First that may not be the problem. And second some actuators require quite a bit of disassembly to access. Looking up labor times may help indicate how big of a job it is. However, I’d recommend having a different shop check it out before sending your husband diving in without a clear idea of what the problem actually is. Good Luck

    Fix More I write all my own material based on my nearly 30 years as a master tech, here’s some more of my articles on our sister site WiseAutoTools.com. More information on car heater systems and how to troubleshoot linked in the bottom auto repair article.

    Recent Posts at WiseAutoTools.com

     

  15. Judy :

    Date: November 14, 2013 @ 5:14 pm

    Can low coolant/freon cause a car heater to blow cold air? My 2002 Jeep Liberty Limited just began to have this problem. Looking for cheap easy fixes to try since I don;t know a reputable mechanic and my funds are limited.

  16. dennisb - Auto Tool Sales :

    Date: November 14, 2013 @ 5:15 pm

    Yes definitely.

  17. Jeffrey W. Puhl :

    Date: November 28, 2013 @ 8:53 pm

    My dodge Ram shows temp of about 210-220. When I turn on the heater I feel nice heat then I hear a pop like a door closing and the temp than turns cold. I think that if I could get to that door I can fix it. Is there an easy way to access it?

  18. dennisb - Auto Tool Sales :

    Date: November 29, 2013 @ 9:17 am

    Jeffrey, unfortunately there’s no easy fix or easy way to get at the blend doors on a Ram. The only way to access those doors is to remove the dash and case. If it was just a recirculate/fresh air door (which it’s not going by your description) we sell an aftermarket door that doesn’t require removing the dash. You just work along the glovebox area. But that is just for low airflow caused by a broken door by the blower motor. Here’s a link to the Dodge Ram article, it may help if you decide to remove the dash. Just keep in mind we don’t sell the doors for the blend door.
    http://www.denlorstools.com/autoblog/2009/11/dodge-ram-low-air-flow-from-ac-vents/

  19. Sharon :

    Date: December 3, 2013 @ 1:26 am

    I have a 2007 dodge caliber. It has started acting as if it wants to run hot. The temp guage goes in red and over heat light comes on. The heater wont work half the time. It will warm up then go right back cold and I’m constantly putting coolant in it. I’ve went through a gallon in one week.

  20. dennisb - Auto Tool Sales :

    Date: December 3, 2013 @ 7:11 am

    Sharon, coolant is the common denominator, required for both the car heater to work and for the engine’s cooling system to work. When it’s low neither will work properly. Even after you add coolant, it’s possible to have air in the system. Once the cooling system issue of losing coolant is resolved the heater should be fixed as well. See our car overheating tips repair blog article for more information.

  21. Al :

    Date: December 6, 2013 @ 12:53 am

    First Dennis thanks for the info!
    Here is my delima. I have a 04 Mitsubishi Galant ES 4 cyl and the car most of the time will blow cold air when you turn the heat on. On rare occasions it will get lukewarm-warm and then cold after about 20 seconds. The coolant in the car was very dirty and we flushed it several times but still no heat. We then flushed the heater core itself and the strangest thing happened. We flushed in both directions. When we flushed in one direction the water flowed freely but when we put the water on the other heater core hose it barely flushed at all(it almost seemed completely blocked.) We didn’t know if this was a blockage or not because we weren’t sure how the water flows in and out of the heater core in an 04 Galant (we assumed it flowed in from the top hose and out from the bottom but we weren’t sure) when we felt the hose when the car was running, the top one was hot and the bottom was luke-warm. The car does not leak inside, nor do the windows fog up with a greasy film so I don’t think the core is bad. So in short we don’t know what’s wrong… I guess my question is should you see water free flowing from both heater core hoses when they are flushed backwards and forwards? (or is there something different in an 04 Galant in how the water flows in and out of the heater core?) Sorry this is so long but I wanted to be as detailed as possible I hope you understand the question. We are really at a lost here and the winter is coming lol!

  22. dennisb - Auto Tool Sales :

    Date: December 6, 2013 @ 7:30 am

    Blowing hot for just a few seconds sounds like a blocked heater core. The heater core should allow water to flow freely in either direction. You could try switching the hoses to see if that would help temporarily. But, if the cooling system is still dirty, even a new heater core is likely to be clogged up quickly. You may end up replacing the radiator and heater core in effort to have a clean system. Sometimes the inside of the engine can get pretty bad too, but since the engine is mostly aluminum on the Galant, hopefully there won’t be much rusty steel to contaminate the coolant. Freeze plugs are steel and I’ve seen them rust all the way through. I would pressure test the system after your repairs, to make sure that all of this flushing hasn’t caused a leak to spring. Good Luck.

  23. katie :

    Date: December 11, 2013 @ 8:05 pm

    My elantra only blows hot air when I’m on the hwy. Does it need to be flushed or is it something more expensive?

  24. dennisb - Auto Tool Sales :

    Date: December 12, 2013 @ 10:31 am

    If the coolant level is full, it’s most likely a partially blocked heater core. Good Luck.

  25. Deb Watson :

    Date: January 26, 2014 @ 2:40 pm

    2009 Nissan Quest. Heater works fine initially, but when we drive long enough for the engine to warm up completely, we get HOT air if we crank the knob all the way up (90) But cold air in any other setting. While under warranty, we had it in several times for this issue. They kept saying they couldn’t reproduce it until we finally had a svc mgr tell us it wasa known issue and they had fixed it. The winter after that, it was fine. Now it’s happening again. My husband and another mechanic both said the vehicle is reading the ambient temp at 180 degrees. They replaced what they thought would fix it. It didn’t. Other thoughts?

  26. dennisb - Auto Tool Sales :

    Date: January 26, 2014 @ 4:17 pm

    Sounds like a possible problem with the control head. Nut, since you know there is a “known” issue specific to your model; I would check with the service dept. at the dealer again to see if they can tell you what the “common problem/failure” is. Good Luck.

  27. Joe :

    Date: February 10, 2014 @ 1:25 pm

    Hi Dennis,

    I just recently bought an as is 02 Jeep Grand Cherokee, The heat blows nice and hot when the truck warms up and has been idling, but when I drive it the temp gage starts to drop and so does the heat, on the highway my temp gage is telling me about 105/110. A buddy of mine suggest putting a small piece of cardboard in front of the radiator,, what are your thoughts

  28. dennisb - Auto Tool Sales :

    Date: February 10, 2014 @ 1:36 pm

    In extremely cold temperatures with the symptoms you’ve described I would suspect a stuck open thermostat. The thermostat is designed to close and stop coolant flow when the engine’s temperature is below normal operating temp. The cardboard would make no change if the coolant is kept from flowing by a properly working thermostat. Usually a CEL (Check Engine Light) will come on and a code will be stored for “engine temperature not reaching operating temp in specified time” but if the problem just started or the code was cleared before you purchased the Jeep, it may take awhile for the fault to set the code. Good Luck.

  29. JL :

    Date: February 27, 2014 @ 5:29 pm

    My 2003 dodge stratus heater is only heating up when I give it gas. If I’m sitting idol it will go cold. Any idea what could be wrong

  30. dennisb - Auto Tool Sales :

    Date: February 27, 2014 @ 5:35 pm

    Low coolant, air in the system or a clogged heater core. Good Luck.

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