Strong musty or moldy smell from the indoor unit when it starts.

Safety first: Turn off power before cleaning.

Steps to try:

• Clean filters: Wash thoroughly with mild detergent and dry completely.
• Use coil cleaner: Apply a no-rinse evaporator coil cleaner designed for AC units to the coil surface.
• Disinfect drain pan: Wipe the pan with a diluted bleach or specialized HVAC disinfectant solution.
• Run in fan mode: After cleaning, run the unit in fan-only mode for 30–60 minutes to dry the coil.

When to call a technician: Severe mold contamination may require professional deep cleaning or coil removal.