Why do windows fog while heater is running?

Asked by Jan 02, 2017 at 05:36 PM about the 1999 Toyota Corolla LE

Question type: Maintenance & Repair

On My Mothers 1999 Toyota Corolla when she runs the heater the windows fog
up in the car.  If she runs the AC at the same time it clears up the windows.  We
don't live close by so I hope to be able to find out whats wrong so I can get
parts and plan the repair the next time I visit.  Thanks for any help!

4 Answers


Foggy of the windows is normal this time of year with the temperature difference. But most likely the heater core is leaking. Have your Mom check if there is a slick film on the windshield and or coolant in the passenger footwell or she smells antifreeze.


Its most likely normal. Turning on the a/c introduces outside air into the mix and takes out some of the humidity. Its physics, humidity, biometric pressure, it can happen to either inside or outside glass depending on circumstances. If it does it while heat is on open a window slightly. If its moisture in system from leak in heater core replace it, you will know if that is the issue because of the coolant smell in cabin.


The AC condenses moisture out of the air which reduces the humidity and fogging.

I told her to let me know if she smelled antifreeze or saw any in the floor. For some reason my Mom won't let me fix anything of hers unless I just do it, She never tells me when something is wrong. Thanks for the help guys, got to keep her informed so she will keep feeding me the symptoms! Any more ideas will be appreciated.

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