The answer to 'central air smells bad' is that there are several common causes, including mold growth, dirty air filters, or issues with the ductwork. This can lead to unpleasant odors circulating through your home. The solution depends on the underlying cause, but may involve cleaning the system, replacing filters, or having a professional inspect and repair any issues.
If the smell is musty or mildew-like, it could indicate mold growth in the air conditioning system. This is a common problem, especially in humid climates, and should be addressed promptly to prevent the mold from spreading. Homeowners may be able to clean the system themselves, but for more extensive mold issues, it's best to call in an HVAC professional.