How To Fix 7 Bad Air Conditioner Smells (Musty, Mildew, Cat Pee)

You kick the AC on and the house starts cooling down, but the air conditioner smells bad when it turns on and you’re wondering if it’s something to be concerned with. 

It’s a common occurrence in many households and can potentially be quite a problem. Although there are several different smells that could be coming from your air conditioner, the most common is when your ac smells musty. It’s commonly called dirty sock syndrome.

While musty air conditioner smells are the most common odor complaint I receive as a licensed HVAC contractor, there are several other smells your system can have. These can indicate different potential issues that will need to be checked out.

Understanding Common AC Smells 

Moving sufficient air is an important part of a properly operating air conditioner. Because of this, even the slightest of smells can be sensed when the AC kicks on. Common smells include:

  • Musty
  • Moldy
  • Cat Pee
  • Mildewy
  • Burning
  • Sulfur or Rotten Egg
  • Sewer 

Musty and Moldy Smells

Musty and moldy AC odors are very typical of a dirty evaporator coil.

Your evaporator coil is the indoor component of your air conditioner. It’s usually very cold and dark and condensation forms on the coil from the warm humid air thats moving through it. Dust and dirt that makes it through the filter collect on the fins and in the drain pan.

This leaves the perfect environment for mold and bacteria to grow. This growth can make your air conditioner smell musty and decrease indoor air quality in the house. Prolonged exposure may increase respiratory issues, especially in individuals with sensitivities and allergies.

Excess water in your condensate pan from a clogged drain line can also make your air conditioner smell moldy. The condensate pan sits in the bottom of the evaporator coil and can collect the same debris your coil gets on it.

RELATED: What Should My Air Conditioner Be Set At?

Cat Pee Or Ammonia

If you smell a cat pee odor, this is also likely related to mold in the HVAC system which can give off strong ammonia odors.

The intense ammonia smell is typically from the mycotoxins that is released into the air from the mold.

Different types of bacterial growth in the duct system can also give off urine-like odors.

Burning Smells

If your air conditioner has a burning smell when you try to run it, there’s a significant problem.

In most cases, your blower motor is probably overheating or you have an electrical issue in your furnace or air handler. If it is your motor that’s causing the smell, it could have a winding issue and may need to be replaced. If it uses a capacitor, the motor itself may be okay, the capacitor may be bad or weak. 

It’s also possible that you have a plugged filter or frozen evaporator coil. The blower motors used today are “air over” motors. They use the air moving through the system to keep them cool. So if it’s not able to move enough air, it can overheat and will shut down.

If you have a burning smell from your air conditioner, it’s best to shut it down immediately. If ignored, you will most definitely have an expensive repair waiting for you.

Sulfur Smell

rotten egg or sulfur smell is a major concern, but not necessary for your air conditioner. 

Natural gas has an additive that smells like sulfur or rotten eggs so we can indicate if there’s a gas leak and shut the gas off to prevent anything from happening.

When your air conditioner runs, it uses the blower from your furnace to move air through the coil and the ductwork. If there’s a gas leak at your furnace, you may get a sulfur like smell every time you run your AC.

This is a smell that will need professional attention immediately to prevent injury or potential damage to your house. It’s best to call your gas company to address this and take the necessary steps to correct it.

Sewer Smell

Every once in awhile you may get a sewer gas smell that seems to come from your air conditioner.

Most of the time it’s coming from your condensate drain line. If the drain doesn’t have a trap or if the trap is dry, it will allow sewer gas to enter the duct and be pushed through the duct.

This usually only happens the first couple of times you run your AC that season. Once the condensate fills the trap, the smell should go away. 

Read Also >> Why Is My Brand New AC Unit Freezing Up?

How to Fix Musty and Moldy Smells

Replace your air filter and clean your evaporator coil to eliminate musty and moldy smells.

Replace Your Air Filter

Remove your old filter and replace it with a new one. Dust, dirt, and other debris on the filter can be a significant source of musty smells from your air conditioner.

Clean Your Coil If Your Air Conditioning Smells Musty

Most of the time there’s a coil door that can be removed to access your coil. Remove the screws and the door and visually inspect the coil.

You’ll usually see significant debris on the aluminum fins. These fins are very sharp and can be bent very easily, so proceed with caution.

For larger debris, I’ve found success using a soft bristled paint brush and a shop vac. Once the large loose pieces are removed, use an evaporator coil cleaner to remove the remaining hard to reach buildup.

Evaporator coil cleaners are safe to use on copper and aluminum and usually require no rinsing. Bleach and vinegar are very acidic and can damage metals, causing corrosion down the road and potentially causing leaks to form. 

Clean Your Drain Pan To Eliminate Mildew Smell In A/C

While you have access to your evaporator coil, you should clean the plastic drain pan at the bottom that collects the condensate and drains it away.

Mold growth is very common here and can make your AC smell like mildew. Use a diluted bleach solution or distilled white vinegar and pour it directly into the pan. Be careful not to get bleach or vinegar on the fins or tubes of the coil, it should go directly in the pan.

Do this several times to flush any excess debris out of the pan. Condensate drain tablets can be placed in the pan to help prevent additional buildup and mold growth in the future.

Read Also >> What Condensate Pumps Do We Recommend?

Identify and Repair Burning Smells

Burning smells from motor failure or other electrical issues can be difficult to repair and you may need to call a professional to take care of this for you.

Determine The Cause of The Smell

As soon as you notice a burning smell, turn the power off to your equipment and locate the source of the smell.

Start by removing the access doors to the furnace or air handler. Visually inspect the wiring looking for burn marks or loose connections. Look at the circuit board in the blower cabinet. This is a common place for burns caused by a power surge or stuck relay.

Check the temperature of your motor, very carefully, with your hand. If the back of the motor is too hot to touch, you have a motor issue.

Try spinning the blower wheel by hand. If it’s stiff or difficult to turn, you have a motor bearing issue. Most motors today have sealed bearings, so the entire motor will need replaced to correct this problem.

If the wheel spins freely, you either have a capacitor problem or a motor winding issue. Once the motor has cooled down you can try turning it back on and carefully spinning by hand. If it begins to run on its own, its most like a bad capacitor. 

If it doesn’t run on its own, there’s a good chance it’s a winding issue and the motor will need replacing.

Read Also >> Why Is There A Moldy Smell In The Air Conditioner?

Prevention Tips

Maintaining your air conditioner annually is vital in preventing these smells.

Keeping your filter clean and clearing your condensate drain line will help stop major mold and mildew buildup from occurring. Either inspect and clean it yourself or hire a professional to take care of it for you.

Use evaporator coil cleaner and condensate drain tablets when necessary to be thorough in eliminating all mold and bacteria. Preventing buildup and smells is essential for good indoor air quality.

Installing a UV light above or below your evaporator coil will help kill mold and mildew and prevent it from coming back. UV is a great solution to removing mold on your evaporator coil and will help increase the quality of the air in your home. 

UV Bulbs

Frequently Asked Questions

Where Should I Start When It Smells?

Start by replacing the air filter and thoroughly cleaning any mold, mildew, and other debris from your evaporator coil. Make sure condensate drain lines are clean and clear and no water is building up.

Will The Bad Smell Go Away On It’s Own?

Most of the time no, it won’t go away on its own. Smells from an air conditioner are signs of a system that needs cleaning or of parts that are failing and need repaired. If you begin smelling something from your air conditioner it’s best to have it looked at.

How Can I Make It Smell Better?

Keeping your evaporator coil clean and replacing your filter regularly will help keep things smelling fresh. Usually evaporator coil cleaners have a clean smell and can help give you that freshness.

Why Does It Smell Like Sulfur?

If your air conditioner smells similar to rotten eggs, you may have a gas leak at your furnace and should call a professional or the gas company to check your gas lines and locate any potential leaks.

Why Is There A Musty Odor?

A musty smell usually means you have a dirty evaporator coil and/or a dirty air filter. Dirty sock syndrome is a common issue when mold and mildew build up on the coil and the odor from that gets blown through the house when the air conditioner turns on.


Whether it’s a musty moldy smell or a burning smell, noticing something different with your air conditioner is important and shouldn’t be ignored.

Keeping filters and coils clean is essential for an air conditioner to function properly and efficiently. It’s also the best way to maintain good indoor air quality and prevent mold growth.

Sometimes it’s easy to overlook a smell because the air conditioner is still working and keeping the house at the right temperature. Even if your system seems to be working fine, don’t ignore a different smell. It’s one of the first steps in identifying a potential future problem. 

If you aren’t able to clean your system effectively, call a professional. They should have the tools and equipment to take care of all of it for you.


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