If your vehicle is showing musty, moldy, or mildew smell from the a/c vents, here are the most likely causes and the parts you'll need to fix each one.
The cabin air filter traps dust, pollen, and moisture from the outside air. When it gets old and clogged, it becomes a breeding ground for mold and mildew, pushing that smell directly into the cabin.
Condensation naturally forms on the A/C evaporator during use. If the system doesn't dry out before the car is turned off, mold and bacteria grow on the fins, causing a 'dirty sock' smell when the fan blows.
The tube that drains normal A/C condensation under the car can get clogged with dirt or debris. This leaves standing water inside the dash, leading to severe mold growth and a persistent swampy smell.
Leaves, pine needles, and dirt can accumulate in the cowl area at the base of the windshield. As this organic matter rots, the HVAC system pulls the musty smell inside.
If a cabin filter is forced or crushed into place during a recent service, it allows damp debris to bypass the filter entirely and rot directly against the evaporator core.
A small leak in the heater core can introduce warm engine coolant into the HVAC box. While coolant usually smells sweet, the constant dampness creates a greenhouse effect inside the dash that breeds mildew.