The answer to 'why is my AC smells bad' is that there are several common causes, including mold and mildew buildup, a dead animal in the ductwork, or a refrigerant leak. These issues can produce unpleasant odors that circulate through your home when the AC is running.
Mold and mildew often grow in the condensate drain line or evaporator coil if there is excess moisture. A dead animal, like a rodent, stuck in the ductwork can also create a foul smell. Finally, a refrigerant leak allows the gas to escape and mix with warm air, producing a sweet, pungent odor.