The answer to 'central air smells bad' is that a foul odor coming from your central air conditioning system is typically caused by mold, mildew, or bacterial growth. This can happen when moisture builds up in the ducts, coils, or drain line, providing the perfect environment for these microorganisms to thrive.
Some common symptoms include a musty, rotten, or sewer-like smell coming from the vents when the AC is running. This is not only unpleasant but can also be a health concern, as the spores and toxins can circulate through your home's air. The solution often requires a thorough cleaning and disinfection of the HVAC components by a professional technician.