The answer to 'air conditioner smells bad' is that a foul odor coming from your air conditioning system is often caused by mold, mildew, or bacterial growth in the unit. This can happen when moisture builds up inside the AC, providing an ideal environment for these microorganisms to thrive.
Some common symptoms of a smelly AC include a musty, rotten, or sewer-like smell coming from the vents or outdoor unit. This odor may be accompanied by poor airflow, reduced cooling capacity, and even respiratory irritation for people in the home. The good news is that there are several steps you can take to diagnose and resolve the issue, though in some cases it's best to call in a professional HVAC technician.