Why Do My Car Windows Get Foggy When I Turn On the Heater?

Fall is here, and the weather is cooling down outside. You turn on the heater on a cold evening and notice that your car windows start to fog up. You switch to the defroster so you can see out of the windshield. This does not help. Rather, the windshield gets foggier. This is a sign that the heater core is leaking and you have engine coolant in the air that is flowing out of the vents. Unfortunately, this makes the air humid and the coolant sticks to the windshield and windows.

Musty Air Odors

Another sign that the core is leaking and you have coolant in the air is musty air odors. You will smell the musty odor in the air that is flowing out of the vents. This odor can also be sweet. Sometimes, the air will smell like maple syrup or candy. Unfortunately, you will end up with foggy windows and sweet-smelling air until you get the heater core replaced.

Cold Cabin Air

Unfortunately, if the heater core is leaking, you won’t get any hot air coming out of the vents. The air flows over the core when it is filled with hot engine coolant in order to heat up. Consequently, if there is no coolant in the core, the air is going to remain cold. No matter how high you increase the temperature, you will have cold air blowing out of the vents.

Low Coolant Level

It might seem strange, but you will have cold air blowing out of the vents and an overheating engine. This is because the leaking heater core causes low coolant levels in the cooling system. Consequently, the engine cannot run at a normal temperature because there is not enough coolant to reduce its temperature. This will leave you with a rising temperature gauge needle.


Eventually, the needle will hit the red zone on the temperature gauge. When this happens, you should pull your vehicle over and cut the engine. It is important that you do not drive your automobile when the engine is running too hot. If you continue to operate the vehicle, you could cause unnecessary and serious engine damage. For example, a common reason why the head gasket cracks is excess heat.

If you are having the problems discussed in this blog post, call our auto service shop today to set up an appointment. We will inspect the heater core and replace it if necessary.

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