How to Diagnose and Repair Your Car’s Heater

Everyone hates this situation. You get into your car on a cold morning, start it up and drive away. About a 1/2 a mile or so later you say to yourself, “Dang, it’s still cold in here. What’s up with the heater?”

You put your hand in front of the vent and it’s like ice.

After letting out a few expletives, you decide to wait.

Two more miles down the road you put your hand in front of several heat vents, not trusting the first one, and nothing but cold air greets your already frigid fingers.

Figure out how to diagnose and repair your car’s heater when you get a chance.

How to Diagnose and Repair Your Car’s Heater if it’s Not Blowing Hot Air

Here, we’ll discuss the steps to diagnosing and repairing your car’s heater in easy, logical steps. This way, you can check the simple fixes first, and if they work, then you’ve yourself saved a lot of time and money.

Step #1.

Check your coolant levels. Be sure the car is off and cool. Never check your coolant level with a hot engine. You could injure yourself because the fluid will be under pressure and will burst out, possibly burning you.

You can have problems even if your coolant level is just a little low. Your coolant system is generally placed a little higher than the other systems, and so a slightly low coolant level can adversely affect your car’s heater.

If your coolant level is low and a cap is directly on the radiator, then fill the radiator using a lisle coolant funnel. This will help get rid of any air pockets in the system. If your cap is not directly on the radiator, then you won’t need the lisle coolant funnel and can fill normally.

One more thing you can do to help burp the air out of the system is to squeeze the upper radiator hose. When you do this, you can see bubbles or burping. This will help ensure you don’t have a hidden air pocket somewhere.

Next: some vehicles have an extra coolant reservoir. Check to see if yours does, and if needed, fill it to the cold fill line that you see on the side of that reservoir.

Step #2.

Go for a drive and let your engine temperature gauge get into the normal heat range.

If you notice that your vehicle is overheating and you’re still not getting heat, then your water pump isn’t working. Generally, a bad water pump will leak and the impeller will still spin. This will prevent the hot coolant from being forced into your heater core or even your radiator, which is the cause of the overheating.

A water pump replacement should really be done by a mechanic. But if you’re pretty handy, follow the steps for replacing it in your vehicle repair manual. Click here to find a workshop manual for your car.

Step #3.

Determine if your engine temperature gauge never gets up to a normal range and you’re not getting heat.

This is most likely caused by a bad thermostat or a thermostat that is stuck open.

To fix the thermostat, just follow the top radiator hose until it meets with your engine. The thermostat will almost always be located between the engine and the hose. There are typically only 2 bolts holding it in place. Remove and replace the thermostat and you should solve that issue.

Step #4.

Check your fan’s speed setting levels. If it works and you’re not getting hot air, then your blower motor resistor is likely shot. This is simple to replace and relatively cheap. Follow the instructions in your vehicle repair manual.

Step #5.

If your fan is not sending out any air at all and you’ve tested it on all the speed settings, then your blower motor is likely bad and needs to be replaced.

Simply follow the instructions in your vehicle repair manual.

There you have it, the most common repair problems easily solved using the above tips.

About Chad Ina



  1. Avatar

    Very good quite simple instructions very clear

  2. Avatar

    You forgot the damned motorized Flaps directing the air flow and or recirculation / fresh air of the ACC.
    If the ACC have divided heat zones rh/lh there might also be a faulty temp.-sensor by one/more outlet that give false readings.

  3. Avatar

    And then there is the “blocked heater core” syndrome. For which there is only one solution.
    How to check?
    …place a hand on the supply or return lines that go from the engine bay through the firewall to the cabin…no flow will equal low relative temperature to the rest of the cooling system.
    How to fix?
    Spend lots of time or money or both pulling the dash apart to get to said core and replace. enjoy

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