Bad Smell From Your AC? What Musty, Burning and Other Odors Mean
A musty, burning, rotten-egg, sour or chemical smell when the AC runs.
By Mark Curant·Updated
Most likely causes
- Musty or mildew: mold or bacteria on the indoor coil, in the drain pan, on the filter or in the ducts
- Burning or electrical: an overheating motor or wiring. Turn the system off.
- Rotten eggs: a natural gas leak (from a gas furnace or another appliance) or a dead animal in the ducts
- Dirty socks: bacteria growing on the indoor coil, common on heat pumps
- Sewage: a dried-out drain trap letting sewer gas in
- Dusty burning on the first heating day of the season: dust burning off, usually gone within an hour
Before you open anything
Turn the system off at the thermostat, then switch off power at the breaker and at the disconnect beside the outdoor unit. Never touch the capacitor, even with the power off: it can hold a dangerous charge. Refrigerant work is for licensed technicians only. Reset a tripped breaker once; if it trips again, leave it off.
If you smell rotten eggs (natural gas), don't switch anything on or off. Get everyone out, then call your gas utility or 911 from outside. If a carbon monoxide alarm sounds, get outside and call for help. Your air conditioner doesn't burn gas, but a gas furnace often shares the same ducts and blower.
Check these first (free, 5 minutes)
- Burning, electrical or rotten-egg smell: follow the safety box above before doing anything else.
- Air filter: a damp, dirty filter can smell musty. Replace it.
- Drain pan and drain line: standing water under the indoor unit grows mold and bacteria.
- Every vent, or one room? A smell at every vent points to the indoor unit; one vent points to that duct.
Causes, signs and repair costs
Mold or mildew on the coil, pan or ducts
DIY or proMusty smell, strongest when the system starts; visible dark growth on the coil or in the pan
Typical cost: $0 (filter, drain) to $100–$600 cleaning
Clogged drain or standing water
DIY or proMusty or sour smell, water in the drain pan
Typical cost: $10–$20 DIY, $75–$200 pro
See: AC leaking water →Dirty filter
DIYMusty or dusty smell, filter gray or damp
Typical cost: $5–$20
Overheating motor or wiring
ProBurning plastic or electrical smell, sometimes with buzzing or a tripped breaker
Typical cost: $150–$800
Dust burning off heating parts
DIYDusty burning smell the first time the heat runs each season, fading within an hour
Typical cost: $0
Natural gas leak (not from the AC)
ProRotten-egg smell near a gas furnace, water heater or other gas appliance
Typical cost: Call your gas utility first
Dead animal in the ducts or unit
ProRotten smell getting stronger, often worst at one vent
Typical cost: $75–$200 service call plus removal and cleaning
"Dirty sock" bacteria on the coil
ProSmell of dirty socks or sweat, often when a heat pump switches modes
Typical cost: $100–$600
Sewer gas from a dry drain trap
DIY or proSewage smell near the indoor unit or a floor drain the condensate runs to
Typical cost: $0 (refill the trap) to a plumber's visit
Refrigerant leak
ProA faint sweet or chemical smell with cooling getting worse; many refrigerants have little smell, so this is a weak sign
Typical cost: $400–$2,400 to find, fix and recharge
| Cause | Signs | Who fixes it | Typical cost |
|---|---|---|---|
| Mold or mildew on the coil, pan or ducts | Musty smell, strongest when the system starts; visible dark growth on the coil or in the pan | DIY or pro | $0 (filter, drain) to $100–$600 cleaning |
| Clogged drain or standing waterSee: AC leaking water → | Musty or sour smell, water in the drain pan | DIY or pro | $10–$20 DIY, $75–$200 pro |
| Dirty filter | Musty or dusty smell, filter gray or damp | DIY | $5–$20 |
| Overheating motor or wiring | Burning plastic or electrical smell, sometimes with buzzing or a tripped breaker | Pro | $150–$800 |
| Dust burning off heating parts | Dusty burning smell the first time the heat runs each season, fading within an hour | DIY | $0 |
| Natural gas leak (not from the AC) | Rotten-egg smell near a gas furnace, water heater or other gas appliance | Pro | Call your gas utility first |
| Dead animal in the ducts or unit | Rotten smell getting stronger, often worst at one vent | Pro | $75–$200 service call plus removal and cleaning |
| "Dirty sock" bacteria on the coil | Smell of dirty socks or sweat, often when a heat pump switches modes | Pro | $100–$600 |
| Sewer gas from a dry drain trap | Sewage smell near the indoor unit or a floor drain the condensate runs to | DIY or pro | $0 (refill the trap) to a plumber's visit |
| Refrigerant leak | A faint sweet or chemical smell with cooling getting worse; many refrigerants have little smell, so this is a weak sign | Pro | $400–$2,400 to find, fix and recharge |
Typical U.S. installed prices in 2026. Most repairs also carry a service call fee of $75–$200, often credited toward the repair. See AC repair costs in 2026 for the full list.
Step by step
- 1
Rule out the dangerous smells
Burning or electrical: turn the system off at the thermostat and the breaker, and call. Rotten eggs: don't touch switches; leave and call the gas utility from outside. Then come back to the rest of these steps.
- 2
Replace the filter
A damp, dirty filter is the cheapest cause to rule out. If the smell fades within a day, that was it.
- 3
Check the drain pan and line
With power off, look in the pan under the indoor unit. Standing water means the drain is clogged: clear it with a wet/dry vacuum on the outside end, then pour a cup of distilled white vinegar into the cleanout. See the leaking water page for details.
- 4
Look at the coil if you can
Many indoor units have a removable access panel in front of the coil. With power off, look for dark growth on the coil or in the pan. Visible mold on the coil is a job for a professional cleaning.
- 5
Refill dry drain traps
For a sewage smell, pour a quart of water into any floor drain near the indoor unit, and into the condensate line's trap if it has one. Traps that dry out let sewer gas back into the house.
Call a technician if
- Any burning or electrical smell
- A rotten smell that keeps getting stronger, especially at one vent
- A musty smell that returns after a new filter and a clear drain
- Visible mold on the coil, in the pan or inside the ducts
Red flags on a quote
- Whole-house duct cleaning sold as the cure for a musty smell before anyone checks the coil, drain pan and drain line
- A UV light or air purifier sold as the only fix for mold, without fixing the moisture that feeds it
- Refrigerant added for a smell, with no leak search
Facing an expensive repair on an older system? Run it through the repair vs replace calculator, or read the $5,000 rule.
Frequently asked questions
Why does my AC smell musty?
Usually because mold or bacteria are growing where moisture collects: on the indoor coil, in the drain pan, on a damp filter or in the ducts. Replace the filter, clear the drain and check the pan. If the smell comes back, have the coil cleaned. Keeping indoor humidity below 60% (EPA's guideline) slows the growth.
Is a burning smell from my AC dangerous?
It can be. An electrical or burning-plastic smell can mean an overheating motor or wiring, so turn the system off at the thermostat and breaker and call a technician. The one usually harmless exception is a dusty smell the first time the heat runs each season.
Why does my AC smell like rotten eggs?
Natural gas has a rotten-egg odor added to it, so treat that smell as a possible gas leak: leave the house and call your gas utility from outside. Your AC doesn't use gas, but a gas furnace often shares its ducts. If the gas company finds no leak, a dead animal in the ductwork is the next most likely cause.
Should I get my air ducts cleaned for a bad smell?
Usually not first. The EPA says duct cleaning isn't something most homes need routinely, and suggests considering it when there's visible mold growth inside the ducts, signs of rodents or insects, or so much dust and debris that it blows out of the vents. Most AC smells come from the coil, drain pan or filter.
Related problems
AC leaking water
Water around the indoor unit, a dripping ceiling or a full drain pan.
House humid with the AC on
The temperature is fine, but the air feels damp, sticky or cold and clammy.
AC making noise
Buzzing, clicking, hissing, banging, squealing or rattling from the indoor or outdoor unit.
Fixed it? Check what it costs to run
If your system is old enough to keep breaking, compare its running cost with a new one before the next repair.
Open the SEER savings calculator