What makes my car AC smell when it first turns on in cool weather?

A sour or musty smell from the vents for the first few minutes on a cool fall or spring day usually means mildew on the evaporator core. Moisture sits on the fins between uses, especially when the AC is rarely used, or run briefly and then shut off, and in Michigan’s humid shoulder seasons, that moisture feeds mold growth.

The smell is often strongest right after startup (customer reports the “AC smells for first minute”) and then fades as the air dries out while you drive. This is common after Ann Arbor’s freeze‑thaw cycles, when the HVAC box never fully dries and organic debris builds up inside.

Hoover Street Auto Repair can clean the HVAC box, replace the cabin air filter, and recommend operating habits to reduce musty startup odors during Ann Arbor’s cooler months.