Why Do My Car Windows Fog Up When I Turn On the Heater?

Your car windows are fogging up when you turn on the heater most likely because your heater core has a leak. The heater core uses hot engine coolant to warm the air when you turn on your vehicle’s heater. If it is leaking, the air is filled with coolant and very moist. When it meets with the air inside your vehicle’s cabin, the windows will fog up and no amount of defrosting will help. Oxford Automotive lists the other symptoms of a leaking heater core.

Cold Air

Naturally, if your heater core is leaking the hot engine coolant, the air will not be warmed up as it flows over the core. Consequently, even though you have your vehicle’s heater on, you will end up with cold air coming out of the vents. This doesn’t help you when it’s cold outside, and, unfortunately, the only way to solve the problem is to replace the heater core.

Sweet/Musty Odors

You will also detect sweet and musty odors in the air that is blowing out of the vents. What you are smelling is the engine coolant. Engine coolant smells like fruit or maple syrup. It has a very sweet and musty odor to it. If you can smell something like this in the air that blows through the vents, this, too, is a sign that your heater core is leaking engine coolant out of its tubes.

Low Coolant Levels

Unfortunately, the leak will reduce the coolant levels found in your vehicle’s cooling system. As a consequence, your engine will overheat even though you’ve got cold air inside. This is frustrating, but the unique situation makes diagnosing the problem easier. This combination of occurrences happens when the heater core is leaking. If you think about it, it makes perfect sense. There is no engine coolant to warm the air and there is no engine coolant in your engine.

Wet Floorboard

Finally, you will find your vehicle’s heater core under the dash on the passenger side of your automobile. As you can imagine, this can create quite a mess if the heater core is leaking. You will end up with engine coolant on the floorboard of the passenger side of your automobile. This, too, makes diagnosing the problem easier. The heater core is the only cooling system part that is found in this location.

Oxford Automotive in Oxford, PA, can help, so call us today. We’d be happy to inspect your heater core if you suspect it is leaking. If it is, we will replace it.

Photo by rvimages from Getty Images Signature via Canva Pro

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