Why Does My AC Smell Bad? A Pro's Guide to Air Conditioner Odors
From musty mildew to burning plastic to that infamous "why does my AC smell like pee?" question — here's how Wisconsin homeowners can identify what's wrong, what they can fix themselves, and when to call an HVAC tech.
Safety first: If you smell rotten eggs, gas, or burning electrical components, shut the system off, leave the house if it's gas, and call a professional before restarting.
Musty / moldy smell (dirty sock smell)
The #1 cause of AC odors. Moisture sits on the evaporator coil and in the drain pan, growing mildew and bacteria. Common after a humid week or when the unit hasn't run in a while.
What you can try
- Replace the air filter — a clogged filter traps moisture against the coil.
- Pour a cup of distilled white vinegar (or a 1:1 vinegar/water mix) down the condensate drain line to clear algae.
- Run the fan on 'On' (not 'Auto') for an hour to dry the coil.
When to call a pro: Schedule a coil cleaning and drain-line flush. If the smell returns within weeks, you likely have biofilm on the coil or a clogged drain pan that needs professional cleaning.
Urine smell ('why does my AC smell like pee?')
Almost always a dead animal (mouse, bird, or bat) decomposing in the ductwork, near the outdoor unit, or inside the air handler. Less commonly, heavy mold colonies on the coil can produce an ammonia-like smell.
What you can try
- Check the outdoor condenser unit for nests or carcasses and clear away debris.
- Inspect visible ductwork registers with a flashlight.
- Replace the air filter and run the fan to flush the system.
When to call a pro: If you can't locate the source, an HVAC tech can open the air handler, inspect the blower compartment, and clean the ducts. Persistent ammonia smells with no carcass = professional coil cleaning.
Burning, electrical, or plastic smell
Overheating motor, failing capacitor, burned wiring, or a fan belt rubbing. The first run of the season can also smell like burning dust — that should fade within an hour.
What you can try
- If it's the first cool-down of the year, give it 30–60 minutes. If the smell stays, shut the system off.
- Turn the AC off at the thermostat AND the breaker. Do not keep running it.
When to call a pro: Call an HVAC tech the same day. A burning electrical smell is a fire risk — don't restart the unit until a professional inspects the blower motor, capacitor, and wiring.
Rotten-egg / sulfur smell
Natural gas leak (gas furnaces share ductwork with the AC) or, less commonly, a dead rodent. Utilities add a sulfur scent to gas so leaks are noticeable.
What you can try
- Leave the house immediately. Do not flip switches, light matches, or use your phone inside.
- Call your gas utility's emergency line from outside.
When to call a pro: After the gas company clears the leak, have a licensed HVAC and plumbing pro inspect the gas line, furnace heat exchanger, and connections. We handle gas line repairs for homes across western Wisconsin.
Skunk or 'chemical' smell
Refrigerant (Freon) leak. Modern refrigerants are nearly odorless, but additives and the reaction with internal components can produce a sweet chloroform-like or skunky chemical smell.
What you can try
- Shut the system off. Refrigerant exposure in a closed space isn't safe.
- Ventilate the area.
When to call a pro: Refrigerant work is EPA-regulated. A certified tech will leak-test, repair, and recharge the system. Running an AC low on refrigerant also damages the compressor — fix it fast.
Cigarette smoke or stale tobacco smell
The evaporator coil and filter absorb airborne particles over time. After cooking smoke, fire damage, or previous smokers in the home, those particles re-release when the AC runs.
What you can try
- Replace the filter with a higher-MERV pleated filter (MERV 11–13).
- Wipe accessible return grilles.
When to call a pro: A professional coil cleaning plus duct cleaning resets the system. Consider adding a media air cleaner or HRV — both we install routinely on Wisconsin homes.
Still smelling something off?
Gunderson Plumbing, HVAC & Concrete services air conditioners, furnaces, and ductwork across Dunn, Eau Claire, Chippewa, Pepin, Pierce, and St. Croix Counties. Same-day diagnosis on most calls.