Heater Is Blowing Cold Air? How to Diagnose the Heat Not Working in Your Car

If you turn the heat on and find the heater is blowing cold air in your car, this guide and video reviews how to check check the cooling system and diagnose it yourself.

How to Diagnose the Cooling System If Your Heater Is Blowing Cold Air

Filling the system with coolant

Steps to Diagnose the Cooling System

  1. Check the Coolant Level with the Engine Cold

    Make sure the engine is cold, and check the coolant level. Look at the level in the coolant reservoir, and check the level in the radiator if it has a cap, and . See if there is coolant inside. This means the problem is not because of a leak or low coolant.

  2. Run the Engine

    Replace cap and run the car for a long time. We ran it for 45 minutes.

  3. Check the Upper and Lower Radiator Hoses for Heat

    Check upper radiator hose and the lower hose for heat. Make sure the vehicle is warmed up.

    Feel the hoses if you don’t have a temperature gauge. Make sure there isn’t a significant difference in temperature. In our example, there is about a 5 degree difference, which is not a problem.

  4. Check the Coolant Gauge

    Check the coolant gauge and make sure its pointing in the middle, indicating it can reach operating temp.

  5. Wait for the Cooling Fans to Turn on

    Make sure the cooling fans turn on

  6. Check the Heater Core Hoses

    One hose enters the heater core and one exits it. Both should be hot. In this example they are cold after running the engine for 45 minutes. If the hoses are cold, there could be an issue with the water pump. In our example the cooling fans turned on, the temperature gauge reads fine, and the engine isn’t overheating, so it’s not likely the water pump is causing the heater to blow cold air since the coolant is circulating. Otherwise coolant won’t cool the engine down and it will overheat.

    If the heater core hoses are warm and there is still no heat in the cabin, there might be an issue with the blend door actuator, which is the part that controls the flow of hot and cold air in the cabin when the heat is turned on. Since there is no hot air to begin with, we know the problem is not with the actuator.

  7. Check the Heater Valve and Make Sure It’s Opening

    Make sure the heater core valve is opening to circulate the coolant. The back of water pump was cool because this valve wasn’t opening.

    We removed the valve with the vehicle off and found stop leak in the system that was clogging the valve.

    To test the heater valve, remove the cable and physically move butterfly with the system hot to see if the heater core hoses heat up. This will let you know there is a problem with the valve.

  8. Check the Heater Core for Clogging

    Check the heater core for clogging by removing the hoses. In this picture, stop leak clogged the heater core, and it was very evident with the clogged hoses.

  9. Flush the Heater Core and Coolant System

    Flush the heater core. It may take some time; it took us three times, but this will remove any debris that might be clogging it. Flush the whole coolant system and drain the coolant. Put harnesses back where you found them so they don’t hit the exhaust and melt. Replace the coolant and bleed the system.

    Read more about how to flush the heater core

    Read more about how to bleed the coolant system

  10. If the Heat Still Doesn’t Improve, Replace the Radiator and Possibly the Water Pump

    If your heater is still blowing cold air after checking these parts, replace the radiator and, in our case, possibly the water pump.

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Heater Is Blowing Cold Air? How to Check the Cooling System - 1A Auto
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Heater Is Blowing Cold Air? How to Check the Cooling System - 1A Auto
If your heater is blowing cold air, this guide has tips from our expert mechanics to help you diagnose the cooling system to find and fix the cause
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