A musty smell usually means bacteria on the evaporator. A burnt smell = electrical problem, act now. Rotten-egg smell = a dead pest or drain issue. Never ignore an AC smell — especially burnt.
See if any of these signs match your device — each one increases the likelihood of this fault.
These are the causes we encounter while repairing Bad smell from AC — ranked from most to least common.
Bacteria and mold on the evaporator from humidity + dust = musty smell
Overheated wire or motor = burnt plastic odor (fire risk)
Dead small animal inside the outdoor unit or ductwork
Old refrigerant leak (R-22) with sulfur content
Stagnant water sitting in the drain pan
Any of the following = a danger signal — call now and don't try to fix it yourself.
On-site diagnosis, clear pricing before any work, and a documented 6-month warranty.