Why does my car AC smell like vinegar?

A sour, vinegary smell is bacteria and mold living on the permanently damp evaporator core — the same thing that causes the 'gym sock' smell, just a different colony. It's worst on cars driven short distances in humid weather, because the evaporator never gets a chance to dry out. Replace the cabin air filter first, then spray an evaporator/HVAC cleaner into the fresh-air intake at the base of the windshield with the fan on high and AC off. Habit that prevents it coming back: switch AC off but leave the fan running for the last two minutes of every drive.

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