Why does my AC smell bad when I turn it on?

That musty gym-sock smell is mold and bacteria growing on the wet evaporator coil — very common in humid climates and cars that mostly do short trips. Fix path: (1) replace the cabin air filter, (2) spray an evaporator cleaner (Kool-It or similar) into the fresh-air intake with the AC on max, (3) run the AC off but fan on for the last minute of every drive to dry the evaporator. If it smells more like sweet syrup, that's a coolant leak from the heater core — different problem, needs a shop.

Get a free AI-ranked diagnosis for your exact year, make and model — with fair repair cost ranges and DIY-vs-shop guidance.