What's That Funky Smell When I Turn on the Air Conditioning?

By Rick Popely, Cars.com

That nasty odor pouring out of your dashboard vents probably means that the air-conditioning system contains an offensive collection of moisture, dirt, debris, mildew or all of the above.

The air-conditioning system removes moisture from the air, and as that moisture collects in the system over time it can lead to mildew, mold and other odor-inducing organic growth. In addition, dirt, rotting leaves and debris of all types also collects in the system, potentially adding to an olfactory buffet that can remind you of your high school locker room.

The evaporator core, for example, acts like a radiator, removing heat from incoming air and causing water to form on its surface. That water is supposed to drip down to a drain pan and exit through a drain tube (that's the water you see under your car after you've used the air).

Mechanics tell us that when we shut off the engine after using the air conditioning, particularly in recirculation mode, moisture remains on the evaporator's fins, helping dust and grime collect on the evaporator. That creates a petri dish for bacteria, mold, mildew and other organisms.

One solution is to spray a disinfectant such as Lysol or one specifically designed for air-conditioning systems into the fresh-air intake (usually at the base of the windshield) for the heating, ventilation and air-conditioning system, though this will likely reduce odors only temporarily. Another is to spray the evaporator with disinfectant and rinse with water, although it is often behind the dashboard and not readily accessible.

Make sure that the air-conditioning drain tube is clear of debris. We've also seen suggestions to run the fan with the air conditioning off before shutting the engine off to help dry out the system and prevent mildew, an idea that seems worth a try.