Safe checks first: musty, sour, or "dirty sock" (the common one)
Cool, damp coil + dust = a biofilm farm. It announces itself at startup when the fan blasts the accumulated odor.
- Clean the filters (rinse, dry fully before reinstalling).
- Run FAN-only for 2-3 hours after heavy cooling days - drying the coil between uses is the cheapest fix there is.
- Use the unit's Active Clean / self-dry function if it has one (CL display on many models is this feature working, not a fault).
- If the smell survives clean filters and dry-outs, the coil and blower wheel need a proper deep clean - a technician service with coil-safe cleaners, typically bundled with a maintenance visit. Skip supermarket "AC bomb" sprays; residue feeds the problem and can corrode fins.
- Check the condensate drain outside - a blocked, stagnant line smells too (see leaking water for the clearing steps).
Beyond the safe checks (technician scope)
Coil and blower-wheel deep cleaning (panel-off, coil-safe chemistry), drainage rework for chronically wet coils, and everything in the two scenarios below.
Chemical, sweet, or solvent-like
Treat as a potential refrigerant leak until proven otherwise: ventilate the room, power the unit off at the breaker, and call a technician. Modern units use A2L-class refrigerants (R-454B, R-32) - mildly flammable, certified-handling only. Some units have leak-detection codes (EH C1 class) - check the library, and mention the code when you call.
Burning or scorched
Power off at the breaker immediately. Dust burning off after a long idle season produces a brief light odor through the first hour - but a persistent electrical/burning smell is a fault. Do not run it again until inspected.
When to stop and call
Any chemical or burning scenario above; musty smell that returns within days of a professional cleaning (points to drainage or a chronically wet coil - a system-level fix, not another spray).