Car Heater Blows Cold – Auto Service Tips

1:52 pm DIY, How To Auto Repair

Heater_Hoses_at_Firewall

Heater hose clamps like pictured can be removed easily with special hose clamp tools.

Summary: © DenLorsTools.com In this auto service repair article, we cover how car heaters work. Knowing how the automotive heater system is designed to work, allows the car owner or auto tech to diagnose problems more quickly and easily. Car heaters are designed much differently than home heaters, therefore diagnosing and repairing auto heaters requires different service procedures. Auto service tips and information provided in this car repair article is written by a master automotive technician that has many years in the field. The info. is provided in simple language that is easy to understand by anyone – experienced in auto service or not. (Be sure to read the questions and answers in the comment section at the end of the article. Also see page 2 of Car Heater Blows Cold for even more tips.)

It’s always best to have a basic understanding on how a particular system works, to be able to diagnose problems easier. When a car heater blows cold air, there are several possibilities that should be considered. Read on to learn more. Many people are unaware that there’s NOT an electric element in a car’s heater system. Heater elements are common in portable heaters used in the home – however automobiles use coolant from the car engine’s cooling system to transfer heat to the passenger compartment, through the use of a heater core. That’s why a car heater doesn’t start blowing warm air until the vehicle reaches operating temperature. The heater core looks similar to a small radiator – it has coils and fins to transfer heat to the air.  The car engine’s coolant is pumped through the heater core while the fan (also called a blower), pushes air through the heater core fins. When air blows through the heater core fins the air is warmed and in turn heats up the passenger compartment. One of the most common causes of a car heater blowing cold air is from low coolant.  As stated before, coolant is what warms the heater core – if the coolant is low, there may not be enough heat transfer from the heater core to the air to heat the passenger compartment. When it ‘s cold outside, low coolant may not cause the engine to overheat right away. If it did cause the engine’s temperature gauge to read hot, the driver would be warned and the  low coolant problem would be discovered more quickly. The point is, when checking the heater be sure the radiator is full of coolant. Once the coolant is verified to be full, feel the heater hoses that go to the firewall. With the engine at normal operating temperature, BOTH of the heater hoses should be hot to the touch. If only one is hot, this indicates there is a blockage in the heater core or there is air trapped in the heater core preventing proper flow.

Removing Trapped Air

Using a Lisle coolant funnel is the best way to remove air from a car’s cooling system. If the coolant is really brown, has been neglected, or if stop leak has been used at some time in the car’s service history, the heater core could be stopped up. The blockage can sometimes be cleared  by removing the heater hoses and using a garden hose with a sprayer to flush the heater core out. If neither of the hoses are hot to the touch, there could also be a malfunctioning heater control valve, if used on the model you’re working on. Check for presence of a heater control valve by following the heater hoses back to the engine. Sometimes, a vacuum line could have a break causing there to be no vacuum to operate the valve. Also it should be noted that if this is your first winter with this car (and you’re unfamiliar with the car’s repair history), the previous owner could have by-passed the heater core due to a leak. When a heater core leaks, the passenger side carpet will become soaked with coolant. This should not be confused with an AC (condensation) water leak. The labor involved to change a heater core is usually several hours depending on the model. This is the reason some people will loop the heater hoses together with a 5/8″ union – by-passing a leaking heater core instead of repairing it properly.

After the previous steps are taken to insure the heater core is in the loop, the coolant is full and hot water is circulating in and out of the heater core, read the following regarding the HVAC (Heating, Ventilation & Air Conditioning) case. Doors within the heater case are either controlled by electric actuators, cables or vacuum motors. Adjust the heat control to both extremes while listening for movement of the blend door. With a cable operated heater door it’s easiest to hear the door thump when it closes. If the door is not operating, find the door that controls the airflow across the heater core. If there is an electric motor that controls the door, tapping on it can sometimes make it work temporarily for testing purposes. A vacuum operated motor needs vacuum to work, so using a hand-held vacuum pump for testing is usually the easiest way to check operation. If the vacuum motor does not hold vacuum, the diaphragm is leaking requiring replacement. To go deeper into diagnosis of the AC control head’s function and diagnosis, specific vehicle repair manual diagrams may be needed.

Next Page

Related Car Repair Article and Mechanics Tools

Car Heater Blows Cold – Part 2 with more Tips

The Lisle Coolant Fill Funnel to Remove Trapped Air

Car Tools – General Automotiove Tools

Shop Heaters and Blowers – Heaters

Mechanics Specialty Tools – Cooling System Service

195 Responses
  1. dennisb - Auto Tool Sales :

    Date: March 8, 2014 @ 10:24 am

    Catalin, not sure which video you’re referring to (not one of mine). But the link to my other repair blog was intended to just show you what the actuator looks like and what it does. I understand that your’s is not making noise. They don’t have to make noise when they fail. Good Luck.

  2. Roman :

    Date: April 5, 2014 @ 12:22 pm

    i have a 2002 chrylser sebring lxi 2.7 litre convertible, about a week ago, it stopped blowing hot air through vents, the temp gauge reads normal, coolant is full, bled system just to be sure… was told it was the a/c control head (w/o auto temp), replaced that, still nothing but cold air. both lines going into heatercore are hot…its a head scratcher…any help would be appreciated?

  3. dennisb - Auto Tool Sales :

    Date: April 5, 2014 @ 2:57 pm

    I would check for codes with the climate control system and voltage readings at the blend actuator when temp setting has been changed back and forth. Good Luck.

  4. DAVID :

    Date: April 11, 2014 @ 5:22 pm

    I have a 2001 Nissan Altima that is due for maintenance this week (scheduled CV joints, new belts, O2 sensors). Now the car seems to have some overheating issues. I replaced the radiator and thermostat 3 months ago because a small crack in the radiator, it seemed to have fixed it for a few months. Now presently there is a slight squealing noise in the AM when I start it to go to work. This was not an issue as I assumed it was the AC pulley with a flat bearing. It would do it every now and again when I turned on the A/C but would sometimes go away if the RPM’s slowed. Now the A/C button can be depressed and the temp gauge rises, even if the A/C button is depressed or not. The A/C compressor still blows very cold and the heater works well as well. The A/C was working for a bit then it started to not work, instead blowing room temp air and the temp gauge was rising to the point of overheating. The heater was turned on to see if it helped cool the vehicle and it did not. My wife noticed the heat gauge rising only when you accelerated to fast or when the gas pedal was not depressed at all. Since I have replaced the thermostat and the radiator I am leaning toward replacing the water pump, should I look into some other cause of the overheating issue?

  5. dennisb - Auto Tool Sales :

    Date: April 11, 2014 @ 5:56 pm

    David, you may have several issues. The squealing is most likely a loose belt. The overheating is most likely low coolant. I would pressure test the cooling system to see what is leaking, The water pump may have a leak or it could be something else. The AC should not blow cold when it’s been turned off with the button. I would do some testing and make sure the clutch is disengaging as it should. Maybe let the tech doing the maintenance work that you’ve scheduled look into these issues. Good Luck.

  6. DAVID :

    Date: April 11, 2014 @ 6:10 pm

    There is not any liquid leaking, I did the pressure test all was good. The A/C does not blow cold when the button is depressed but did stop blowing cold when the temp gauge beagn to rise to an overheating temp.

  7. dennisb - Auto Tool Sales :

    Date: April 12, 2014 @ 9:56 am

    David, thanks for clarifying. It’s possible that there’s a cooling fan issue. If the AC condenser (similar to the radiator but for the AC) is not getting enough airflow the AC will not work well (maybe just when driving down the road). And if the cooling fan is not working the coolant in the radiator may only get enough airflow to keep the engine cool when driving at highway speeds. Good Luck.

  8. Kimberly :

    Date: May 23, 2014 @ 6:44 pm

    I have a 2010 Chrysler Sebring. The A/C will only blow out cold air if is parked in a cool location prior to driving. Secondly, the air and heat cut off when you push the control to “high” or “max” out put. Do I need a new compressor? Or does this sound like an electrical problem?

  9. dennisb - Auto Tool Sales :

    Date: May 24, 2014 @ 9:30 am

    Kimberly, sounds like two different problems. I would check the AC gauge readings for the first problem of the AC not cooling well. See our AC gauge readings explained article for information on what the readings mean. The second problem has nothing to do with the compressor. It sounds like an electrical issue as you mentioned. I’d get an electrical wiring diagram for your vehicle and start testing circuits to pinpoint that problem. Good Luck.

  10. Wise Auto Tool Blog » Water Leaking Into Car Floorboard – Wetting Carpet :

    Date: August 1, 2014 @ 8:20 am

    […] of clear water then the heater core is likely the culprit. See the following repair article on Car Heater Blows Cold Air for more on that […]

  11. Mike :

    Date: November 19, 2014 @ 10:22 am

    Dennis,

    Im bringing this back from the dead persay, but I have been blessed with working on my buddies 00 Sebring Convertible. (fun..) and we did a ton of work to it just before winter hit. We replaced his thermostat because the car was overheating to the point that he had to shut the car off all the time. Now, it fixed that issue. We replaced the thermostat with a OE temp one from Autozone. Now he just called me and told me he is having the “Heater works when idling but driving it cools off” issue. Do you think we got a bad thermostat, or could there be a new issue? 2.5 Ltr V6 just incase. Thanks!!

  12. dennisb - Auto Tool Sales :

    Date: November 19, 2014 @ 10:27 am

    I’d make sure the cooling system is full and there’s no air in the system. Good Luck.

  13. catherine :

    Date: January 1, 2015 @ 11:04 am

    My 1996 pontiac Bonneville only has heat when I add more gas

  14. dennisb - Auto Tool Sales :

    Date: January 1, 2015 @ 3:00 pm

    Catherine, adding gasoline has no relation to the car’s heater unless you’re topping off the cooling system’s overflow bottle while you’re at the pump. Good Luck.

  15. bob :

    Date: January 2, 2015 @ 4:22 pm

    Hi Dennis,

    I have an 01 Sebring sedan with 2.7l and the heater isn’t blowing warm air. Both heater core hoses are hot and the engine is not overheating. What is the likely cause?
    Thanks

  16. dennisb - Auto Tool Sales :

    Date: January 2, 2015 @ 4:35 pm

    Normally just low coolant and maybe air in the system. But if both hoses are very hot, perhaps there’s an actuator problem controlling the blend door. The core could be partially restricted also. Good Luck.

  17. brittni :

    Date: January 8, 2015 @ 9:04 pm

    I have a 1999 Ford Taurus an just put a new Thermostat in because the heat didn’t work. Now it only blows hot air when I reach 4000 rpms an when this happens the temperature gauge drops to cold otherwise it blows cold air…help?

  18. tiegua anderson :

    Date: January 23, 2015 @ 5:44 pm

    On my Dodge Neon when I turn on the heat it blows out cold air. Then I got it fixed. It blows out heat when im driving it but when I’m at a stop light, it get blows out cold air again. I know some cars do that but not all cars and I want to know the problem and what can I do to get this fixed. It’s a 2000 Dodge Neon.

  19. dennisb - Auto Tool Sales :

    Date: January 26, 2015 @ 8:03 am

    Since it didn’t stay fixed; I’d guess that there’s a coolant leak that is affecting the heat again. Low coolant is the most common cause of a car heater not working properly. I’d check the coolant level. If it’s low I’d pressure test the system and look for coolant leaks. Then repair the leaks. If leaks are not fixed, just filling the radiator (cooling system) will only be a temporary solution. Good Luck.

  20. Bryan :

    Date: February 6, 2015 @ 11:06 am

    I have a 2003 Jeep Liberty it blows warm air if you use settings 1 and 2 but once you go past that using 3 and 4 it starts to blow cold air any ideas?

  21. dennisb - Auto Tool Sales :

    Date: February 6, 2015 @ 11:17 am

    When on the heater setting, it’s normal to a point for the air to be warmer on slower fan speeds. I’d check all the normal things like coolant level, air in the system etc. Good Luck.

  22. harry :

    Date: February 13, 2015 @ 9:26 pm

    I have a 2004 Sebring, v6. When I shut the car off today I noticed some smoke coming from under the hood. Also the heater only got warm when the gas pedal was depressed. Had the car towed home. The tow truck driver said, I had a small antifreeze leak? I did see that it was slightly wet next to the resovoir?? Big problem??

  23. ernie :

    Date: February 14, 2015 @ 8:12 am

    I have a 99 mustang convertible 3.8 and the heater will blow hot air for the first 20 minutes but once the temperature gauge gets to about mid it stops blowing hot and blows cold the water in the radiator also gets cooler please help…..

  24. dennisb - Auto Tool Sales :

    Date: February 14, 2015 @ 10:19 am

    In extreme cold weather. it’s possible that an incorrect temp thermostat could be the problem. The engine should be running at around 210-212 degrees. THis is best for the engine and the heater system. Good Luck.

  25. dennisb - Auto Tool Sales :

    Date: February 14, 2015 @ 10:21 am

    Low coolant can cause the heater not to work properly. I’d pressure test the system and pinpoint the leak. Could be a small problem or a big problem depending upon what’s leaking. Good Luck.

  26. Raul Alfaro :

    Date: March 11, 2015 @ 11:16 pm

    Grand Cherokee 2011. My A/C blower speed is varying from max to min without any control. Originally, passenger side air was hot while driver side was cold, so the dealer diagnosed low level of refrigerant and they recharged it. After that worked, the blower began to fail, and what’s more, now it is not heating when hot is selected. Any idea? They already replaced the resistor and the blower motor without any result. The dealer diagnosed the lack of heating as a failure in one of the gates (doors), that sounds logical, but they are lost figuring out what is causing the blower speed variation. I wonder if both failures could be related.

  27. dennisb - Auto Tool Sales :

    Date: March 12, 2015 @ 7:01 am

    Some variation in blower speed sound can be normal depending on how much electrical load is on the charging system. I would check the charging system and battery for starters. The electrical connections for the blower and resistor are a source of problems sometimes, so I would check to see if they appear burned (or overheated) which would indicate a poor connection that can affect performance. After the gate (door) has been replaced I would recheck the system to see if you’re still experiencing any issues. If so I would look at the climate control data with a scan tool while the remaining issues are present. Good Luck.

  28. Gillian - SouthNZ :

    Date: August 16, 2015 @ 3:40 am

    Hi Dennis, I am hoping you can help. My husband and I have recently purchased a 1991 Mazda Parkway bus which has been converted into a motorhome. The heater in the bus has warm air coming out ok through the window demister vent (non AC)but we only get cold air through the passenger and driver vents. A new thermostat has been installed recently and the coolant flushed and replenished. This makes for very cold drives in the South of South Island of New Zealand. Any advice would be greatly received. Many thanks

  29. dennisb - Auto Tool Sales :

    Date: August 16, 2015 @ 9:45 am

    Could be a partially blocked heater core or air control door/actuator problem. Unfortunately this problem would take some hands on to check out, which puts me at a disadvantage. Good Luck.

  30. Ekpadi :

    Date: August 23, 2015 @ 11:27 am

    Control to select vent/floor heat/defrost does not change air blowing position. Stuck on vent no matter what position the selector is on. How can I trouble shoot. Is there a vacuum hose or cable that may have broke or come loose under the dash? Or do I need to replace the controler

  31. dennisb - Auto Tool Sales :

    Date: August 24, 2015 @ 9:13 am

    I’d check a repair manual for how your specific vehicle/HVAC controls are set up and go from there. Each type of vehicle are different. Good Luck.

  32. Ricky Espinoza :

    Date: August 24, 2015 @ 10:06 pm

    I have a 2003 Ford Mustang. I hadn’t been using my AC because it would leak water inside the car on the front passenger side below the dash. I thought it to be the heater core, but the other day I bypassed it. Well to my surprise its still leaking water. What could the problem be?

  33. dennisb - Auto Tool Sales :

    Date: August 25, 2015 @ 7:19 am

    Ricky, coolant is usually green NOT clear. Coolant also has a sweet odor. A coolant leak from the heater core will leak all the time not just when you’re running the AC. Condensation from the evaporator is what you have from a stopped up drain or a leaky seal around that drain. Here’s several articles I’ve written on the subject.
    AC Water Leaks Inside Car – How to Fix
    Water Leaking Into Car Floorboard – Wetting Carpet

  34. Will :

    Date: December 6, 2015 @ 1:50 am

    I have a 2009 Nissan Altima both the heat and air went out at the same time. The fan controls work but the temp. of the air coming out Of the vents is the same on both the heat or ac setting. Any suggestions

  35. dennisb - Auto Tool Sales :

    Date: December 6, 2015 @ 9:07 am

    Could be a blend door actuator. If it does not move the door from the middle position, the air could stay pretty much at at one temp. I’d check that and the heater and AC pressure readings to diagnose. Good Luck.

  36. Jose herrera :

    Date: December 11, 2015 @ 4:27 pm

    I have a 1999 Dodge Ram 1500 and the heater blows cold air. I have flushed the heater core and radiator and replaced the thermostat and still no hot air. Help please.

  37. dennisb - Auto Tool Sales :

    Date: December 14, 2015 @ 8:41 am

    I’d check to see if both heater hoses are hot. If not the heater core could still be stopped up some even though you flushed it. Good Luck.

  38. Alberto :

    Date: December 17, 2015 @ 9:42 am

    Hello. Before at all sorry for my english (I am from Spain).
    I have a 1999 Jeep Grand Cherokee (WJ) and my heater core only blows of warm air during a few seconds. 2 pipes of the radiator are warm and the blend doors work well. Also I have cleaned the radiator with a hose and the water was circulating without problem. Is it possible that the heater core is comunicated internally?

  39. dennisb - Auto Tool Sales :

    Date: December 17, 2015 @ 9:48 am

    Most likely the heater core has a build-up inside causing it not to transfer the heat properly. Sometimes flushing is not enough. Good Luck.

  40. Alberto :

    Date: December 17, 2015 @ 4:16 pm

    I will try to dismantle the dashboard and to change the heater.
    Thank you.

  41. trish :

    Date: January 10, 2016 @ 9:20 am

    Hi. I have an 08 Elantra. sometimes will only blow hot air when pressing gas, when idling or stopped blows cold. temp is fine, also when vents are on face and its on heat (ac works fine) windows fog up. any idea?

  42. dennisb - Auto Tool Sales :

    Date: January 11, 2016 @ 8:36 am

    Windows fogging up may indicate a leaky heater core. I’d pressure test the system and go from there. Good Luck.

  43. Dawn :

    Date: January 14, 2016 @ 9:22 am

    I have 00 jeep grand Cherokee and i only have heat at idle and its not really hot but while driving i have no heat changed thermosta but still no heat what should I do

  44. dennisb - Auto Tool Sales :

    Date: January 14, 2016 @ 9:25 am

    Most likely low coolant or air in the system…

  45. dennisb - Auto Tool Sales :

    Date: January 21, 2016 @ 10:52 am

    I have provided as much info in my 2 part car heater articles, including my responses to comments as I can without checking cars in person. My responses to comments have become redundant for our readers answering similar questions repeatedly. In fact it’s likely just about everyone’s questions have been covered previously. If unable to diagnose your heater problem it may be time to get help from a technician in your area. FYI – We don’t offer phone support for troubleshooting heater issues. In order to better serve our customer’s, our toll free phone numbers are only for placing tool orders. Thanks to everyone who has helped make this article a success!

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