Car Heater Blowing Cold Air

Reader Question

Hello!  My name is Mary and I am from Upstate New York area. It’s getting cold up here and today my car heater was blowing cold air at me and it’s too cold for that.  What makes the heater hot and what do you think happened to mine?



Hello up there Mary.  Yes, it’s starting to get cold here in Houston as well…but luckily my car heater is working today. :)

The first thing that comes to mind here Mary, is there is not enough hot coolant/antifreeze inside the radiator and engine to make your heater hot. There is a small aluminum radiator looking part inside the dashboard (called a heater core) of your vehicle.

The hot coolant from the engine circulates inside the heater core inside the dash and a small fan blows air across the heater core and that hot air is directed to you inside the cab of the vehicle.

If the radiator and engine are low on coolant, there is not going to be any hot coolant to circulate throughout the heater core.

So, first thing to do is check the coolant level inside the radiator and top off as needed. Second, if you do have to add a large amount of coolant there must be a coolant leak somewhere that will need to be found and repaired. A cooling system pressure test your mechanic can do is what should be done next.

If the coolant inside the radiator is full, then you might have a problem with the thermostat which is not opening and closing properly…thus hot coolant is not circulating throughout the engine. With the engine hot and running, check the temperature of the upper and lower radiator hoses to make sure they are both hot.

There are two other rubber water hoses that usually run to the back passenger side of the vehicle under the hood. Both of those smaller hoses should be hot as well.

If the radiator and heater hoses are hot, and there is plenty of coolant inside the radiator then I can only assume you have a problem inside the dash of the vehicle. Either a control head problem (the actual control buttons on the dash for the heater and A/C) or you have a problem with the “blend door” operation that diverts the hot air from the heater and the cold air from the air conditioner.

I have seen pencils fall down inside the dashboard and wedge this blend door in the air conditioner position and the door can not move to the heater selection position. Rare, but I have seen it.

The blend door is controlled either with vacuum from the engine, or from an electrical motor inside the dashboard. Either case, you will need to have a mechanic look at this for you since it is not something the lay person is going to be able to do themselves.


Austin Davis


About the Author:

Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."