When your AC kicks on and something smells off, it's worth paying attention. Different odors point to different problems — and a couple shouldn't be ignored. Here's how to read them.
Musty or moldy smell
The most common AC odor. It usually means mold or mildew growing in the system — often from a clogged drain line or moisture sitting in the ductwork. Besides the smell, it affects your indoor air quality. Clearing the condensate drain, changing the filter, and a duct cleaning often solve it.
Burning or electrical smell
A burning, plastic, or electrical smell is a "turn it off now" situation. It can mean an overheating motor, wiring problem, or electrical short — all potential fire risks. Shut the system down and call a professional before running it again.
Rotten egg smell
If you smell rotten eggs or sulfur, treat it as a possible natural gas leak — leave the home and call your gas utility immediately. (More common with heating, but any gas smell is an emergency.) Once safe, have your system inspected.
Dirty-sock smell
That distinct "dirty gym sock" odor comes from bacteria on a damp evaporator coil — common when the AC cycles on and off. A coil cleaning and fresh filter usually fix it.
Dusty smell on first start
A brief dusty smell when you first run the AC after months off is usually harmless — dust burning off. If it lingers, change the filter and have the system checked.
Improve your air for good
Persistent odors often point to air-quality issues. Thermo Master offers duct cleaning and indoor air quality solutions across Palatine and the NW suburbs. If a smell concerns you, reach out — especially for any burning or gas odor.