The answer to 'why is my central air smells bad' is that there are several common causes, including mold growth, stagnant water, or a dead animal in the ductwork. A musty, moldy, or rotten odor from your air conditioning system can indicate a serious problem that requires prompt attention.
Mold growth is one of the most common culprits. Moisture buildup in the HVAC system, often from a clogged drain line or leaky evaporator coil, can lead to mold and mildew that produce foul odors. Stagnant water in the condensate drain pan or ductwork can also cause bacterial growth and smells. Additionally, a dead animal, such as a rodent, trapped in the ductwork will create a very unpleasant odor as it decomposes.