A musty or mildew smell from your Volkswagen's A/C vents is almost always caused by mold, bacteria, or fungal growth inside the evaporator housing, the part inside your dashboard where the air gets cooled, and it's one of the most common A/C complaints, especially in humid climates like Ann Arbor.

When the A/C cools the air, it pulls moisture out of it. That moisture collects on the cold evaporator coils and is supposed to drain out through a small drain tube onto the ground under the car. When that drainage works properly, the system stays relatively dry. But when the drain tube gets clogged, or when the car is turned off while the evaporator is still wet, moisture accumulates inside the dark, warm housing, creating ideal conditions for mold and bacteria to grow. The smell you notice when you first turn on the A/C is those microorganisms being blown directly into your car's cabin.

Contributing factors for musty A/C smells in Volkswagens:

  • A clogged evaporator drain tube that holds standing water inside the housing is a known issue on several VW models;
  • A dirty or long-overdue cabin air filter that accumulates organic debris and moisture, becoming a source of mold growth itself;
  • High humidity conditions are typical of Ann Arbor summers, which means more moisture is extracted from the air on every cooling cycle;
  • Infrequent use of the A/C, which allows moisture to sit rather than drain fully;
  • Leaving the A/C running in recirculation mode for extended periods can increase interior humidity.

One common contributor specific to Michigan weather: drivers who use the A/C heavily all summer and then stop using it in September leave the evaporator housing damp through the fall without ever drying out, giving mold months to grow before the following spring. A good habit is to switch off the A/C but leave the fan running for the last few minutes of your drive, which helps dry the evaporator before the car is parked.

A qualified shop can clear the drain tube, treat the evaporator with an antimicrobial cleaner, and replace the cabin air filter. This typically resolves the odor. If the smell persists, the evaporator housing may need a more thorough cleaning, or the drain tube may need to be physically cleared of debris.