Diagnostic: Why Is the Heater in My Car Not Working


The heater in your car, truck or SUV depends on several components in order to work properly. When one or more of these start to fail, the performance of your heater and defroster can diminish considerably. You can wind up dealing with a heater that only blows cold, or in some cases, doesn’t blow at all.

Common Signs & Symptoms

My Car's Air Isn't Blowing At All

My Car's Heater Speed Won't Change

My Car's Air is Blowing, But it Isn't Warm


Common Signs of Heater System Problems

Basic heating and cooling system maintenance is important, which means making sure your antifreeze, or coolant, is at the proper level. Your vehicle’s manufacturer will recommend the type of coolant you should use, the ratio of coolant to water, and the ideal interval between each coolant flush. If any of these factors are ignored, they can lead to other problems.

If you’ve noticed your heater isn’t working for one reason or another—or isn’t working well—there are a number of possible causes.
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Heater Symptom 1 – My Car’s Air Isn’t Blowing At All

checking a car fuse

If you can’t get any air to blow at all, it’s not specifically a heater issue - it may be a fuse. Check the fuse first, and replace it if needed.
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blower motor coming out from underneath the car dash

If it’s not the fuse, in many cases a blower motor is also a fairly straightforward replacement that you can do yourself. The blower motor is the fan that helps move air into the cabin of your vehicle.

Do It Right: In how to replace your blower motor with our step-by-step How-To video.

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Heater Symptom 2 – My Car’s Heater Speed Won’t Change

blower motor resistor connectors

When your heater blows warm air, but you can’t adjust the speed of the fan, it’s likely your blower motor resistor. The blower motor resistor controls the fan speed of your blower motor. Thankfully, installing a new blower motor resistor is also typically a simple repair.

Do It Right: Learn how to replace your blower motor resistor with our step-by-step How-To video.

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comparing old and new cabin air filters

Another possibility, especially if you have poor air flow or notice odors from the vents, is a clogged cabin air filter.

Do It Right: Learn how to replace your cabin air filter with our step-by-step How-To video.

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Car AC Fan dial turning the air up

There can also be issues with a heater’s blend door or actuator, which control not only how air is routed into the cabin—including settings like floor, defrost or re-circulation—but also the ability to fine-tune the specific temperature of the air coming through the vents.


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Heater Symptom 3 – My Car’s Air Is Blowing, But It Isn’t Warm

pouring antifreeze

If you’ve got a situation where air is blowing, but isn’t warm, start by checking your coolant level. If it’s low, there may not be enough to flow from the engine to the heater core, where warm air is blown into the cabin of your vehicle. One of the concerns if your coolant level is low is the possibility of a leak. Keep an eye out under the front end of your vehicle, especially if you’ve had to top it off more than once. Low coolant levels can quickly cause your engine to overheat, which may cause additional damage.
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Car Antifreeze gauge reading cold

Another possible cause for a heater that won’t blow warm air is a malfunctioning thermostat. Your thermostat opens and closes to regulate the temperature of the coolant inside the engine. Until the engine warms up, the thermostat should remain closed, which helps your engine and heater warm up. If the thermostat is stuck in the open position, you might notice that the gauge stays on “C” for a long period of time, or it may just never seem to reach its normal operating temperature. Coolant in the heater core below this temperature may not be warm enough for normal heater operation.
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Under the car oil drip on pavement

Other possible causes are a bad heater control valve or heater core, which can both cause the air not to blow warm or to blow warm only intermittently. The heater control valve controls the flow of coolant to the heater core when the heater or defroster is turned on. When it’s defective, the valve may not open to allow the coolant into the heater core, and it may leak coolant. If your coolant level and heater core are both in good shape, the heater control valve could be the problem.
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View of car side mirror from inside car with condensation and rain on the glass window

If your heater core has gone bad, started leaking or become clogged, it can cause a heater not only to blow cool air, but other symptoms like: fog in the cabin of the vehicle, a fruity or sweet smell, overheating, or using far more coolant than normal. Often, the cause has to do with coolant not flowing the way it should through tubing or through the heater core itself, and can even cause coolant to appear on the interior floorboard. If leaks have developed, the heater core should be replaced. If the heater core is clogged, it may benefit from a flush that can remove obstructions and return it to normal operation.

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Additional Resources

Vehicle Air Filter

How to Change Your Air Filter
how to replace blower motor

How to Replace a Blower Motor
Changing cabin air filter in car

How to Change a Cabin Air Filter

The automotive repair work depicted in this series is performed under the direction of the manufacturer of the product featured. Prior to undertaking any of the demonstrated repairs on your vehicle, we suggest you consult with a certified mechanic or another professional who can adequately advise you of the proper repair or remedy required. O’Reilly Automotive Stores, Inc. and its affiliates (“O’Reilly”) disclaims any responsibility for injury or damage resulting from a viewer’s attempt to recreate the repairs shown in this series.