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    Car Heater Not Working? Troubleshooting, How to Fix Repair Tips

    Car Heater Troubleshooting TipsDennisB WiseAutoTools.com © Summary: Easy to understand master tech tips on how to troubleshoot a car’s heater system that’s not working. It’s best to understand the basics of how it works and what makes it blow hot air. That way you’ll know how to fix your car heater if it’s blowing cold air. To put it simply, a car’s heater system transfers heat from the engine to the inside of the car.  It does this by circulating the engine’s coolant from the engine and then through the heater core. Once the hot coolant, which was warmed up by the engine reaches the heater core, air is forced through the exterior of the heater core fins and tubes. Some of the heat is transferred from the coolant in the core to the air which in turn warms the air for the passenger compartment. We provide free auto repair tips on finding and fixing problems with car heaters that are blowing cold air out the dash vents. So if your car’s heater only blows cold air out the vents, read the following car repair article to gain a better understanding on how the heater system works and how to make it blow hot again.

    Engine

    Combustion engines produce a byproduct of heat. Heat is generated due to friction and the small explosions that occur in each cylinder which powers our cars. “Explosions” may sound extreme but that is what occurs when spark from spark plugs ignites the fuel with the precise amount of oxygen, in each cylinder. Without the cooling system working properly the motor would overheat and be damaged. Some of this byproduct; heat from the engine transferred by the circulating coolant is used to as a benefit to heat the passenger compartment.

    Thermostat

    It’s a common misconception that if a car’s heater system is not working, the thermostat must be bad. It is true that if the thermostat does not open to allow coolant flow the heater will not work. But the more immediate issue would be that the car engine would overheat which is a much more serious problem than the passenger compartment being an uncomfortable temperature. Only in very cold weather could the thermostat cause the heater NOT to work. If the temperature gauge shows “cold” even though the car has been running 5 minutes or more, it is possible the t-stat is stuck open. But, the thermostat in a car is not like the thermostat in a house’s AC/Heater system. The car’s thermostat does not regulate the temperature of the coolant specifically for the heater. It is designed to prevent the coolant from flowing until the coolant reaches a certain temp. In most cases (unless it’s very cold outside) once the thermostat is open it stays open. Again, this is not for the heater system, it is for the engine. To bring the temperature of the engine up to peak operating temperature quickly when first started. Also in very cold weather, it also helps maintain the best temperature for the engine. When the engine is at it’s optimum temperature it is more efficient. It runs smoother, gets better mileage because the fuel atomizes better so there’s less waste.

    Water Pump

    The water pump simply pumps the water throughout the cooling system. The waterpump hardly ever just “stops pumping”. In rare cases the impeller may corrode to a point that is not able to pump. If it gets to that point the engine will be overheating. Most of the time when a waterpump does fail, it leaks coolant out of a weep hole because of  a leaky seal. Sometimes they will get noisy from a failed bearing. I could go into more detail on other common failures of water pumps, but for the purposes of understanding how the heater system, works; it’s important just to know that the waterpump is what forces the coolant through the system.

    Heater Core and Radiator

    The radiator is what transfers the engine’s heat to the outside air, in the process cooling the water that get’s recirculated back through the engine cooling it. The heater core is like a miniature radiator. On a small scale it helps cool the water in the cooling system also. But it’s purpose is to heat the passenger compartment like mentioned above. Radiators that are clogged can cause a car to overheat. A heater core that is clogged can prevent the heater from working. Don’t assume that if the heater doesn’t work that the heater core is just bad. The heater core can leak or it can get clogged, but it doesn’t just stop working. One tip to troubleshoot a potentially clogged heater core and to diagnose the problem is to;  feel the two heater hoses that go to the heater core tubes at the firewall. One should be very hot and the other should be very warm. If one is hot and the other is cold, there could be a clog or air trapped in the heater core. Since most of the time the heater core is at a higher elevation than the rest of the cooling system, low coolant can cause the heater core to empty into the lower parts of the cooling system. This will result in no air being warmed and therefore no hot air to be blown through the dash vents. If the coolant level is full and there’s no air in the system, a clogged heater core is the possible problem. Sometimes a heater core can be flushed out, by removing the heater hoses and spraying a garden hose through the heater core tubes. The clamps on the heater hoses may be the old type of screw clamps or they might be spring clamps that are much easier to remove if you have the proper hose clamp spring pliers. On many cars, trucks and vans it’s harder to use regular pliers because of the tight proximity the hose connections, in relation to the firewall and engine. Heater hoses can become almost melted to the heater inlet and outlet pipes. This because the rubber get’s so hot they can just stick to the tubes. *Caution – Forcing the hoses off by prying can damage the heater core. The heater hoses can be loosened by using a hose pick, being careful not to poke a hole through the hoses when working the pick under them. Once sufficiently loosened the hoses can be pulled off without damaging the heater core. These tools can be used whenever the heater hoses need to be removed, if just doing a flush or if changing the heater core.

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    In some older cars, since a leaky heater core can be so labor intensive to change; owners will sometimes bypass them rather than pay to have a heater core changed. Tip: to check for a bypassed heater core look to see if the heater hoses are looped together and clamped by a union rather that going to the heater core pipes/tubes at the firewall. The heater core tubes may just be sticking out of the firewall with no hoses at all or hoses that have been cut off. Labor to change a heater cores varies greatly due to different makes and models. I’ve changed heater cores on older cars in a half an hour, but I’ve also changed heater cores that require the entire dash to be removed to access them. The hardest ones I’ve ever changed were in a Mitsubishi Diamante and one in a Chevrolet Corvette. A 1/4″ impact wrench/driver saves a lot of time when doing dash work yourself. Check the labor hours required for your model with a local shop. They can also let you know what the going shop labor rate is in your area. Usually shop labor rates are higher at the dealerships and lower at independent repair shops. Either way, just make sure the core really needs to be changed. Try the other fixes first and leave replacing the core as a last resort, like if you have coolant puddling on the passenger floor board or running underneath the carpet. If the core has been bypassed at sometime in the past, you can be assured it’s a leaker.

    Heater Control Valve or HVAC Door

    The amount of heat is usually controlled one of two ways. Some cars have a heater control valve (usually by the firewall in line with a heater hose) which works like a sink faucet. The heater control valve opens allowing the flow or closes preventing the flow. If no heat is desired the flow of coolant can be turned off completely. Some cars don’t have a heater valve. In those cars that don’t use a heater valve, the desired amount of heat is controlled by a door that opens and closes in the heater case. This is usually called an HVAC (Heating, Ventilation & Air Conditioning) case. It houses the evaporator, the heater core and has numerous doors that direct airflow to the proper vents (mode control) and adjusts the blend of cold and hot air so the desired temperature that comes through the vents. On a lot of cars these airflow/control doors are made of a plastic composite type material that tend to break over time which can cause blend control or mode issues. Also actuators, which are vacuum operated or little electric motors that open and close the doors can fail also. Sometimes when the electric motors fail, they will strip a plastic gear which will result in a rapid clicking noise when the AC or heat is first turned on. Other times they may continue to click indefinitely. Jeep Grand Cherokee’s dual zone problem is common, resulting from broken doors that can cause one side to work but not the other. The Dodge Ram truck fresh air door problem, is a broken door issue primarily with their fresh air door that can prevent airflow from the blower because it can fall and block the fan.

    Low Coolant – Leaks in the Cooling System

    What we commonly call coolant or anti-freeze is combined with water. The chemicals and additives are designed to increase the boiling point and reduce corrosion inside the cooling system/engine. There are different types of coolant on the market today, not just green. Refer to a service manual to find the correct type for your car. Some aftermarket coolants are OK for use in all models. Some need water added and some do not. Regardless of what type of coolant is being used, any leaks in a cooling system are not only going to reduce the vehicles ability to keep it’s engine cool; it will also affect the performance of the heater system. In fact the loss of heat through the vents may be noticed first, before any overheat problems with the engine occur. Sometimes low coolant, even if it’s just a little low, will make the heater blow cold air out the vents. Sometimes the heater will only blow warm when driving and blow cold at idle. If the coolant is low, use a coolant pressure test kit to pressurize the cooling system that will help reveal any leaks. Some cooling systems have a pressurized coolant overflow reservoir. You can tell if the bottle has what looks like a radiator cap instead of a plastic screw cap or pressed on cap. *Caution – regardless of where the pressurized cap is, don’t open it when the engine is hot. The hot coolant can vaporize and spray causing burns. Avoid scalding yourself or anyone standing nearby by only removing pressurized coolant caps when the engine is cold.

    Air Trapped in the Cooling System

    If air is trapped in the cooling system, especially in the heater core, the heater will not work. Some vehicles have bleeder screws in the cooling system that will allow air to be bled off of the system. Many newer cars will route the lines so they are self bleeding. The use of a coolant funnel is helpful when the pressurized coolant pressure cap is located on the radiator. If the pressurized coolant cap is located on the overflow bottle the funnel will not be of any use as far as bleeding the air from the system. If used on the radiator though, it is very effective. The Lisle coolant funnel is not just to keep from spilling coolant. It comes with many different adapters that fit the top of the radiator in place of the radiator cap. It holds coolant ready to go down into the system and fill the void when the air burps out. By running the car with the funnel in place, I will sometimes squeeze the upper radiator hose to help the air burp and rise. You can actually see the air bubbling to the surface in the coolant funnel. Again, if the pressurized cap is on the reservoir the funnel won’t be effective. The reservoir acts the same as the funnel does on those type of systems. Some professional technicians like to use a coolant vacuum fill method to insure there’s no air in the cooling system

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    Comments

    Comment from Juan Miguel Mendoza-vargas
    Time September 17, 2015 at 8:52 pm

    Thanks for your tips i will do a few tests before. I have to change the heater core

    Comment from Eddie
    Time December 27, 2015 at 10:36 am

    No mention of the radiator cap? Heater in my car would not heat at idle, only when moving.. replaced old cap with new ($3.67) and now heats at idle. It’s like a new car.

    Comment from Dennis Bandy
    Time December 28, 2015 at 9:48 am

    Thanks for the comment. Sometimes the rubber gasket can get out of shape and prevent coolant form re-entering the system from the overflow reservoir. This results in coolant leaving the system and entering the reservoir but not going back in. This is basically the same as the system being low and air in the system. The same would be true if the radiator cap just simply leaks also. That’s why you can’t always determine the system is full just because there’s coolant in the bottle.

    Comment from Don
    Time December 28, 2015 at 4:03 pm

    I have a 2001 dodge ram 2500 cummins it does not get to running temp unless I am going freeway speeds and a son as I allow down the temp drops. My heater does not work unless I am at running temp. Could this be because it may not have a thermostat?

    Comment from Dennis Bandy
    Time December 28, 2015 at 4:37 pm

    It’s possible that the thermostat is stuck open or opening too soon. Unless it’s very cold outside the temp should rise after it’s driven awhile even with a thermostat problem. Of course if it’s very cold outside the thermostat could make a difference. Good Luck.

    Comment from Troy Van Ess
    Time December 30, 2015 at 7:18 pm

    I have the special funnel for burping coolant system. The question I have is the coolant in funnel stays cold as I run the jeep for 1/2 hour or more. I also hear water noise under passenger side when I start my jeep cold. I was thinking air in system but u can’t get any air out. What am I missing?

    Comment from Dennis Bandy
    Time December 31, 2015 at 7:27 am

    Keep in mind the coolant funnel doesn’t help if the reservoir has a pressurized cap. I’d try raising the idle and squeezing the upper radiator hose repeatedly to try to burp the air out. If this works you’ll see air bubbling out. If there are any leaks air can get re-introduced. Also if there’s any head gasket leak as well. Good Luck.

    Comment from Valerie
    Time December 31, 2015 at 4:29 am

    I’ve done multiple tests’ changed the thermostat. Noticed, I’m going to call it the return hose. Has no water or pressure in it. To be more specific, there are two hoses going into the core. Outside one has warm water in it, the inner is the prob..I think.

    Comment from Dennis Bandy
    Time December 31, 2015 at 7:29 am

    Air in the system or plugged radiator core. Good Luck.

    Comment from Mike
    Time January 2, 2016 at 9:34 am

    I have a 99 f150 if its around 35 degrees outside it heats fine but the colder it gets it blows cold air. I have put a new thermostat in it plenty of fluid I have caps on hoses coming in and going out of heater core both have hot water passing through them I put the heater core on 2 yrs ago it’s worked great until this yr?any ideas would be greatly appreciated thank you

    Comment from Dennis Bandy
    Time January 2, 2016 at 2:39 pm

    Possibly air in the system.

    Comment from Josh
    Time January 9, 2016 at 5:37 pm

    I have a 95 Nissan altima. It has new thermostat in it. Both hoses for heater core have coolant flowing through them, and I’ve tried burping the system, but still no heat. The vehicle has electronic temp control ranging from 65-85. The air coming out of the vents is much colder than the air outside and its about 40° outside. Any idea would be appreciated. Also the temperature gauge inside the vehicle does not work, not sure if this could be a cause or not.

    Comment from Dennis Bandy
    Time January 9, 2016 at 7:53 pm

    I’d check to see if both heater hoses are hot before looking into other potential problems. Good Luck.

    Comment from Cilene
    Time January 12, 2016 at 9:53 am

    2003 KIA Sorento- in Houston climate-winter temps around 50 degrees. 1 1/2 years ago car over-heated and towed to my insurance recommended independent shop. Replaced radiator & all anti-freeze. One year to the day, I checked the reservoir and it was empty. Took it back and they replaced a heater hose and the plenum gasket, & throttle body gasket. Turned on the heat and it is blowing outside air. Cover on the engine so I cannot feel if the hoses to the firewall are hot. No fluid leaks on the passenger side and the temperature gauge measures right in the middle when warmed up. I suspect the mechanic did something when replacing the heater hose and gaskets. Am I correct? The heater gauges are digital and the fans are working.

    Comment from Dennis Bandy
    Time January 12, 2016 at 10:38 am

    Most likely air in the system if the coolant is full. I would let a different shop check it out if you don’t trust the original place. They will be able to remove the engine cover and fell the hoses etc. Good Luck.

    Comment from John Lawing
    Time January 12, 2016 at 7:08 pm

    I have a 1990 Toyota Camry le v-6 I have no heat. I had heat last winter. Moved to Texas and thermostat stuck resulting in a blown radiator literally a rip in the top seam. Replaced with brand new one. New radiator cap and thermostat. Coolant full. Been burped. Both hoses get hot neither seems to be hotter than the other. Still no heat. I have read the suggestions numerous times and believe I have attempted all of them. Don’t know what to do next

    Comment from Dennis Bandy
    Time January 13, 2016 at 7:15 am

    The remaining possibilities are (1) a heater core that cannot be flushed. It may be coated inside not allowing proper heat transfer. Or (2) a door issue that allows the air to go through the heater core. This could be from a bad actuator that controls an HVAC door in the dash. Since you’ve tried the simple steps perhaps it’s time to take to a shop that can do some more checking. Good Luck.

    Comment from John Lawing
    Time January 12, 2016 at 6:45 pm

    I have a 1990 Toyota Camry le v-6 and I tried all the suggestions. Both hoses get hot it doesn’t appear as if one is hotter than the other. I have a new radiator. New radiator cap and a new thermostat. Coolant full and burped and still no heat. Had heat last winter. Had to replace radiator due to stuck thermostat. Haven’t had heat since. I am at the end of my rope and don’t know what to do next.

    Comment from kara
    Time January 18, 2016 at 11:39 pm

    I have a 01 voyager…heat was going in and out..in when driving….out when idle…now its blowing straight cold air…temp gage rises….then will fall…it has pretty cold here past few days (19 degrees as high)….

    Comment from Dennis Bandy
    Time January 19, 2016 at 7:46 am

    You may want to check the coolant level. A thermostat that opens too soon, especially in really cold weather may be the problem also. The engine temp should stay at around 210-212 degrees for the heater to work properly. I’ve heard of people in extreme cold temps to partially block airflow for the radiator, with cardboard, so it doesn’t cool off as quickly. Of course this could cause overheating issues when the weather warms up. Good Luck.

    Comment from Stephen
    Time January 21, 2016 at 6:34 pm

    Heater is only blowing warm air when the Rpms are up? The hose going into the heater core is hot but the return is only warm for a minute then it gets hot and then warm again? Thermostat is working fine and coolant level is full. Shouldn’t both hoses going to and from the heater core be the same temperature? The feed stays hot but the return fluctuates from warm to hot. Could this be why my heater is blowing hot?

    Comment from Dennis Bandy
    Time January 22, 2016 at 7:35 am

    Higher RPM’s pumps water at a higher pressure. Core could be blocked partially. I’d try flushing the heater core directly by taking the hoses off and forcing water in alternating pipes using a garden hose. Good Luck.

    Comment from Gass Josef
    Time January 27, 2016 at 1:18 pm

    I’m 78 and been working on cars and scooters since 16…but never have I seen comments like the Mr. Dennis Bandy …thank you you helped me fix my problem…

    Comment from Dennis Bandy
    Time January 27, 2016 at 2:27 pm

    Hey, glad we helped!

    Comment from Len M.
    Time February 12, 2016 at 4:16 am

    2001 Jeep Cherokee XJ blowing cold air. Top hose feels warm but return hose feels cool. Coolant seems full and thermostat warms to 210-220. Do I have a clogged heater core or air in the system? How to clean out the heater core with out using a garden hose? Can I use jugs of water and a compressor to blow out heater core?

    Comment from Dennis Bandy
    Time February 12, 2016 at 10:52 am

    Not sure. No garden hose? Sounds like a fun experiment!

    Comment from GERMAN
    Time March 9, 2016 at 1:31 pm

    Will bypassing the heater core cause my Jeep to turn off ? The jeep turns back on immediately. I’ve recently changed the battery, radiator and distributor.

    Comment from Dennis Bandy
    Time March 11, 2016 at 8:01 am

    Bypassing the heater core will definitely NOT cause your engine to turn off. There are other things that could cause stalling. But, sometimes when changing a battery, the system will have to “relearn” the fuel strategy over again. As time goes on and more carbon builds up in the throttle, the computer changes it’s fuel air mixture to optimize performance. This is because, when carbon builds up on the throttle plates, the volume of air entering the engine if slightly affected. When a battery is replaced, the computer may forget the strategy or settings that were adapted previously. After running for awhile it will adjust. I would go ahead and clean the throttle plates. This may help to remedy the problem a little faster. Good Luck.

    Comment from Lyndsey Rogers
    Time March 15, 2016 at 9:30 am

    I have had my Thermostat and heater core replaced. My engine is over heating when idle and still no heat. I replace my battery before all of that. Seems like since I replaced the battery, all is falling apart!

    Comment from Dennis Bandy
    Time March 16, 2016 at 8:59 am

    Lyndsey, the battery would have nothing to do with the cooling system. A loose belt could possibly cause the water and alternator to turn slower than normal (if they share the same belt on your model). Besides checking belt condition and tightness, I’d also check for air in the cooling system. Good Luck.

    Comment from monzie
    Time March 21, 2016 at 5:06 pm

    I have a 95 Mercury grand marquis and my heat works when I first cut the heater on then 2 minutes later it blows cold air. I flushed out the heater core and changed the thermostat

    Comment from Dennis Bandy
    Time March 22, 2016 at 7:13 am

    It’s eiether air in the sysetm or the heater core is still restricted. Good Luck.

    Comment from justin
    Time March 23, 2016 at 6:14 am

    I have a 2001 Jeep Grand Cherokee with a 4.7. It started running hot when driving and now since that started I have no heat either. Coolant level good, have a new radiator and thermostat. What am I missing? Motor still runs fine no check engine light. What do I do?

    Comment from Dennis Bandy
    Time March 23, 2016 at 7:22 am

    You may have two problems or one issue that’s causing two problems 1. heater not working. 2. engine overheating. I’d check radiator fan operation. I’d check for a blown head gasket. Carbon (exhaust) in the cooling system could cause air pockets which could prevent the heater from working properly. I’d pressure test cooling system because any small hose clamp leaks can allow air in the cooling system. Good Luck.

    Comment from Kevin Urias
    Time April 8, 2016 at 6:44 pm

    Hi I have a toyota celica 02. No heat is coming out while idle or driven, temperature needle work fine, no overheating. I flush the heater core, replace the thermostat, replace the radiator cap. Still no heat. Any Help will be appreciate. Thanks

    Comment from dave smith
    Time May 4, 2016 at 2:36 am

    Hi everyone, I have a Mazda 6 I-sport 2008. My AC is blowing out what looks like smoke and leaves a small puddle of water under my car on the passenger side What can it be?

    Comment from Dennis Bandy
    Time May 4, 2016 at 7:06 am

    As long as the AC is working properly, a water dripping under the vehicle from condensation is normal. Have you ever seen water dripping from your home AC evaporator drain? Same thing. In fact you’d have no water dripping if the AC was not working as it should. As far as the vapor that’s coming from the dash; that’s just got to be moisture in the air. You’d know what smoke smells like and if it was a refrigerant leak, the AC would not be working. The higher the humidity, the more condensation causing the drip underneath the car. And the more likely you may see a little vapor inside the car occasionally.

    Comment from Mike
    Time November 20, 2016 at 5:35 pm

    My 2004 grand marquis not blowing warm heat, already replaced thermostat. Don’t know what to do. Can’t afford expensive fix. Please help me. ?

    Comment from James Mohler
    Time November 29, 2016 at 8:42 am

    The heater in the car hasn’t been working and last night I replaced a blown fuse for one of the three system fans. Now the heat works on the driver side but not on the passenger. The other fuses appeared to be okay but should I still test them? Also there’s a loose wire under the dashboard by the glove box and I’m not sure if that could be the issue. It’s a 2006 Chevy Impala.