Itâ€™s probably mold in the ventilation system. First of all, make sure your condensate drain is not clogged. Next, run the ventilation system with outside air. Keep in mind that turning the temperature dial fully counter-clockwise (i.e. the coldest setting) will switch to inside or â€œrecirculatedâ€? air, so if you commonly do this, make sure you back it off (clockwise) one click to get outside air. Finally, get a can of Lysol and spray the ventilation system with it to kill the mold. You can do this either by removing the cabin air filter and spraying through its opening or by spraying through the intake between the windshield wipers with the fan on full speed and outside air.
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