Every car air conditioner pulls moisture out of the air as it cools. That water condenses on the evaporator, a small radiator-like core tucked behind the dashboard, and is supposed to drip out through a drain tube under the car. On a humid afternoon you can often see a small puddle of clear water under the passenger side after driving; that is normal.

Problems start when the evaporator stays damp after the car is shut off, or when the drain is slow. Dust, pollen and organic debris stick to the wet fins, and microbial growth sets in. The next time the fan starts, it blows that stale, musty air straight into your face. Colorado's dry air helps many cars dry out between uses, but short trips with the A/C running, summer thunderstorms and parking in a humid garage all keep evaporators wet.

Evaporator smell or something else?

Clue
When it smells
Points to evaporator or ducts
Strongest in the first minute of A/C, then fades
Points elsewhere
Constant, even with the fan off
Clue
Where it smells
Points to evaporator or ducts
Right at the dash vents
Points elsewhere
Near a footwell, the trunk or a seat
Clue
Weather link
Points to evaporator or ducts
Worse after humid days or rain
Points elsewhere
Worse when the sun heats the cabin
Clue
Other signs
Points to evaporator or ducts
Weak airflow, dirty cabin filter
Points elsewhere
Damp carpet, fogged glass, visible stain
Clue
Recirculate test
Points to evaporator or ducts
Smell remains on recirculate and on fresh air
Points elsewhere
Smell changes with windows open or closed only

Things to try, from easiest to most involved

  1. Replace the cabin air filter

    On most cars it sits behind the glove box. A clogged filter restricts airflow and holds moisture and debris right in front of the evaporator.

  2. Dry the system before parking

    For the last few minutes of a drive, turn the A/C compressor off but leave the fan running on fresh air. This helps dry the evaporator before the car sits.

  3. Check the drain

    If you never see water dripping under the car after A/C use on a humid day, the drain may be clogged. A mechanic can clear it; in some cars it is reachable from underneath.

  4. Use an evaporator treatment

    Foaming evaporator cleaners are applied through the drain tube or a duct. Follow the product directions carefully and ventilate the car afterward.

  5. Get a professional ventilation treatment

    If the smell persists, a vehicle odor professional or mechanic can treat the evaporator and ducts more thoroughly.

When the musty smell is a sign of something bigger

  • IfThe passenger footwell carpet is wetThenThe evaporator drain may be backing up into the cabin. Have it cleared before cleaning the carpet.
  • IfThe windshield fogs with a greasy film and the air smells sweetThenThat suggests a heater core coolant leak, not mildew. See a mechanic soon.
  • IfThe smell returns within a few weeks of every treatmentThenAsk a mechanic to look at drain flow, door seals and the cowl area below the windshield for leaks.

General information for Aurora, not a quote or professional advice. In an emergency, call 911.