Unpleasant smell from air vents
An unpleasant smell from air vents when the heater or air conditioner is on is a common problem that creates discomfort in the cabin. This usually indicates a buildup of bacteria, mold, or debris in the car's ventilation system, so it's best not to delay solving the issue.
Likely causes — most common first
Dirty cabin air filter
The smell often appears because the filter is clogged with dust, leaves, and organic debris that start to rot from moisture. Also, an old filter lets less air through and contributes to window fogging.
Bacteria and mold on the air conditioner evaporator
The cold evaporator (air conditioner radiator) constantly forms condensate, which in the heat becomes an ideal environment for the growth of fungi and microorganisms that create a musty smell.
Clogged air conditioner drain hole
When the drain tube, through which condensate should flow out under the car, gets clogged with dirt, water accumulates in the heater core. This moisture stagnates, rots, and gives off a strong damp smell.
Foreign objects entering the air ducts
Small leaves, fluff, or insects can enter the ventilation system through the external grille under the windshield. As they decompose inside, they start to smell unpleasantly when the fan is turned on.
