The answer to 'air conditioner smells bad' is that there are a few common causes, including mold and mildew buildup, dirty air filters, or issues with the condensate drain line. The symptoms may include a musty or unpleasant odor coming from the AC unit. To address this, you'll want to first check and clean the air filter, and inspect the condensate drain line for any clogs or backups. If the smell persists, it may indicate a more serious issue like mold or bacteria growth that will require professional HVAC service to properly diagnose and resolve.
It's generally recommended to call in an HVAC technician if the smell is strong, persistent, or you're unsure of the cause. Trying to clean out mold or other contaminants yourself can be risky if you don't have the proper equipment and expertise. A professional will be able to safely identify the source of the odor and provide the right solution.