The answer to 'AC smells bad' is that a foul odor coming from your air conditioning system is usually caused by mold, mildew, or bacteria buildup. This can happen when moisture accumulates in the ducts, coils, or drain lines of your AC unit. The smell may be musty, rotten, or unpleasant.
Some common causes include a clogged condensate drain, dirty air filters, and poor airflow. Neglecting regular maintenance can also allow these problems to develop. If the smell is severe or persistent, it's best to have a professional HVAC technician inspect and clean the system to resolve the issue.