Why Does My Air Conditioner Smell?

Although today’s air conditioners are fancier and more efficient, they are still made up of the same basic components developed by the engineer, Willis Carrier, in 1902. An evaporator, compressor, condenser, and expansion valve manage refrigerant and move the air in two directions: indoors and outside. Window air conditioners contain these components in a small metal box that installs into a window opening. The hot air discharges from the back of the unit, while the condenser coils and a fan cool and re-circulate indoor air. Central air conditioners share the ducts of the home’s heating system, and the compressor and condenser are in a housing located out of doors.


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When something malfunctions in the AC system, our first instinct is to try and fix the problem ourselves. After all, we are perfectly capable of changing an air filter, clearing away obstructions from vents, and stopping a rattling noise by tightening a screw. However, some air conditioning problems are more mysterious and frequently require some professional intervention to resolve.

For instance, you turn on your unit, and the cool air is accompanied by an unpleasant smell. It’s not exactly the result you anticipated, right? OK, you know enough to check the condensate line, for obvious obstructions. But if that’s not the issue, it’s best to contact a reputable AC repair company. The odor could be an indication of more serious problems, like mold or damaged duct work. Characterizations of these odors run the gamut from rotten eggs, to cat pee, smelly feet and beyond. But luckily, noticing these variations actually make finding the cause much easier. The more specific your description, the easier the problem will be to diagnose. Here’s a breakdown of the range of smells you may come across and the kind of problems they may be pointing to in your system.


One of the most common smells reported by customers is a moldy odor. This can mean that mildew or fungus has taken hold and is growing in, or somewhere near, the air conditioner. Along with cooling, one of the functions of an air conditioner is to remove excess moisture. Mold grows in areas where this moisture hasn’t fully drained. A dirty air filter can also contribute to the smell of mildew. It’s likely that a full cleaning will be enough to remove this smell.

Mildew on a wall

Dirty Socks, Stinky Feet, Cheese and Cat Pee

These four distinct, but similar, odors are often indicators of the same problem. It’s known as “Dirty Sock Syndrome” in the HVAC community. This describes a foul, stinky odor often emitted when you first turn on your AC at the beginning of the season. Frequently, it is a result of irregular maintenance that allows dust and mold to build up inside the unit and grow. Again, accumulation of old, dirty water, or a clogged air filter, can also be the culprit, or there may be build-up on the condenser, or evaporator, coils. Your unit will need a thorough cleaning with a solution especially formulated to kill these opportunistic bacteria and protect the coils. One wise step to take before calling a technician, however, is to search for any sneaky cats hanging around, or near, the vents for your unit. Occasionally, truth be told, the smell of cat pee is just that, cat pee.

Man with stinky shoe

Rotten Eggs

A dead animal is frequently the culprit when you detect that unmistakable stench of spoiled eggs coming from the air vents of your AC system. Injured birds or rodents sometimes climb into air conditioner ducts for shelter during the winter months. Sometimes they may die there and begin decomposing, resulting in an acrid odor. Have your ducts examined by a pest control company for animal remains and then professionally cleaned.


This odor could very well be the result of a sewer line that has backed up or ruptured near the duct system for your AC. What you smell is actually methane gas produced by the sewage that has moved into the duct work and is being blown throughout your home. Left unchecked, this could become a dangerous problem. So, be sure and call a technician, or plumber, to check out your system immediately.

Electrical, Burning, or Gunpowder

Electrical smells can indicate that a part inside your unit has malfunctioned, causing the motor to overheat and the insulation on the wires to melt. The smell of burning, or gun powder, can also point to mechanical failure, or a shorted out circuit board. A malfunctioning fan motor can also emit this kind of odor. The situation can be particularly serious if you detect the smell of smoke. Other causes might be that some plastic debris has gotten inside the unit and is melting on the coils, or that a dirty air filter has caused your unit to overheat.

Gas or Skunk

Skunks don’t usually choose to die in or around ductwork, so a decomposing critter is probably not the culprit here. Unfortunately, this kind of stink may indicate that there is gas leaking somewhere near your system. The methyl mercaptan produced by gas, closely resembles the smell of skunk spray and may be entering your house through the duct system. This is a real danger, so don’t fool around. Open your windows, get out of your house, and call the gas company.

However, there are some odors that can smell just like gas, but are actually caused by dust that settles on the heat exchanger and burns off when you turn on the unit for the first time in the season. You will likely detect this gas-like smell coming from the supply vents, but it shouldn’t build up and tends to dissipate as you run the unit.

We’ve tried to cover a range of reasons that your air conditioner may be blowing out air that smells funky and unpleasant. Most of them are easily diagnosed by an HVAC professional and may only require a thorough cleaning to fix. Other repairs may involve replacing some parts and, therefore, be more expensive. Once fixed, however, regular maintenance will help to prevent many of these problems, before they get out of hand and end up causing an unwelcome aroma in your home.

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