Why Does Your Air Conditioner Stink When You Turn It On?

Written By Lester Mclaughlin
Updated On

Are you wondering why your air conditioner stinks when you turn it on?

You’ve come to the right place!

In this Blue National HVAC guide, you’ll learn:

  • The six common causes of bad AC smells
  • How to fix the AC smells for good
  • If bad AC smells are dangerous


Air Conditioner Smells Bad?

And much more!

So, if you’re looking for reasons why your air conditioner smells when you turn it on, keep reading our detailed guide below to get answers to all of your questions!

6 Common Air Conditioner Odors

If a foul odor is coming from your air conditioner or heat pump when you turn it on, you’re probably in a love-hate relationship with your cooling system. You hate that it smells terrible, and you love that it cools your home. A terrible smell also means the house has bad indoor air quality, which isn’t good for your health. 

Well, you don’t have to live with the bad smells forever! Here are the six common air conditioner odors, their causes, and how to fix them. 

Musty, Mold and Mildew Smells

A musty smell or moldy odor coming from your air conditioner when it’s running is not a joyous occasion. The smell may remind you of an old, leaky basement.

If your air conditioner smells like mold or mildew, that is a clear indication that fungus or mold can be cultivating somewhere in your air conditioning system. 


Mold, mildew, and fungus need moisture to grow. The most common areas in the AC unit with high moisture are the evaporator coils, condensate pan, and drain line. When the evaporator coils remove heat from the air, they also remove humidity.

The water in the humid air condenses on the evaporator coils then drips into the condensate drain pan. If the evaporator coils are cover in dirt and dust, they can trap moisture and harbor an excellent environment for mildew and mold and block airflow. From the evaporator coils, the mold can spread to other components, such as the filter. 

Additionally, a clogged drain line can back up and create standing water which is also a great place for mold, mildew, and fungus to grow. 

How To Fix

A mold and mildew smell can be fixed by cleaning the evaporator coils, flushing out the AC drain line, and cleaning or replacing the air filter. 

The evaporator coils should be cleaned with a coil cleaner spray that removes mold and mildew. If the condensate line is backed up, the clog will need to be cleared and treated with a one-to-one mixture of water and bleach. 

Regular air conditioning maintenance can prevent mold and mildew problems from occurring in the first place. If you’d like to learn about our maintenance plans or if you need help removing mold from your air conditioner, give our HVAC professionals a call today!

Rotten Eggs or Sulfur Smell

If there’s a rotten egg smell or, more precisely, the smell of sulfur when you turn your air conditioner on, get out of your house and call emergency services. 

Sulfur odors are almost always a sign of a natural gas leak. In fact, natural gas is entirely odorless- but for easy detection and safety, mercaptan is added to provide its recognizable smell. It smells bad for your protection. 


A gas leak is generally caused by a damaged gas line or a faulty gas connection, valve, or burner in the HVAC system or stove in your home. When the gas leaks, it can be sucked through your HVAC system and distributed throughout your home.

How To Fix

After you and your home’s occupants have safely evacuated your home, wait outdoors for the fire department to assess the situation. In the meantime, open as many windows and doors as possible to let the gas out of your home and turn off the gas supply at the meter. 

If your gas meter does not have a shutoff valve, call the gas company. Alternatively, the fire department can do this for you. 

A gas leak has to be prepared by a certified specialist, which your gas company or the fire department can recommend. 

Oil Smell 

An oil leak usually causes a noticeable oil smell from your air conditioner. Yes, specific components of the air conditioner have oil in them, including the compressor and the two blower motors on the indoor and outdoor fans. 

The oil helps keep these mechanical components lubricated. They each have metal moving parts that rub against each other and wear down if they don’t have lubrication. 

When there’s an oil leak from the blower motor or compressor, it will be easy to identify through the precedence of black, liquid oil. 


An oil leak is usually caused by worn-down fittings, loose connections, failed gaskets, or a broken compressor or motor. Oil leaks are usually accompanied by strange sounds from your AC unit, like screeching or grinding noises. 

How To Fix

The oil smell coming from your air conditioner can only be fixed by fixing the oil leak in your AC system. Certified HVAC technicians should only repair oil leaks to find the actual root cause and proper fix. Give our experienced HVAC pros a call today if you need assistance. 

Electrical Burning Smell

If you’ve ever smelt burnt electronics before, you’ll be able to identify this unique smell immediately. Typically burning electrical smells should be addressed immediately, especially if noises accompany them, the circuit breakers trip, or the AC does not turn on at all.


The burnt electrical smell usually comes from a shorted control circuit board, shorted/grounded wires, or a burnt-out fan blower. Then when your AC turns on, it spreads the electric smell throughout the home. 

How To Fix

When there are shorting and other electrical issues causing noticeable smells, there are usually other issues with the AC system that come along with it. For instance, the central air conditioner may not blow air, might not cool the air, or might not turn on at all.

This smell needs to be inspected and repaired by certified HVAC professionals. Once the cause of the smell is fixed, it should go away on its own within a few days. 

Cigarette and Cigar Smells

If you smoke in your home, the smell will linger, and the smoke will be sucked through your return air vents and recirculated into other rooms of your home. However, the cigarette and cigar smoke smell could be lingering in your home from a previous owner. 


Unless someone is burning cigarette-scented candles, these smells are always caused by smoking in the home. 

How To Fix

Your air conditioning filter, the evaporator coils, and the ductwork can all absorb cigar and cigarette smoke. Depending on how often someone smoked in the home, it can buildup and linger for a long time. 

To remove the cigarette smoke smell from your AC unit, clean or replace the air filter and while you’re at it, clean the evaporator coils, your air duct registers, and even inside your ductwork too. 

Sweaty Gym Clothes Odors

Nobody wants their home smelly, like their sweaty gym clothes and sneakers. If this odor is coming through your air conditioning system, you need to fix this issue right away. 


Sweaty shoes and gym clothes odors can be caused by too much moisture in your AC unit and possibly standing water in your condensate pan. Usually, it’s an issue from too much moisture, and rarely it’s actually dirty gym clothes or smelly shoe odor circulating through your vents. 

How To Fix

Similar to eliminating mold and mildew smells from your AC unit, you’ll want to clean your air filter, evaporator coils, drain pan, and look for clogs in your AC drain line. 

Are Bad Smells From An Air Conditioner Dangerous?

Not all of these terrible smells from your air conditioner are immediately dangerous. However, a few can be harmful – electrical odors can be the start of electrical fires, and the rotten egg/sulfur smell is likely a natural gas leak.

These two smells should be approached and handled with an abundance of caution and by professional repair services

Other odors, such as mildew and mold smells, are not as urgent but can negatively impact your respiratory health over the long term. In fact, the US EPA recommends safe guidelines for indoor air quality to ensure the well-being of home occupants. 

Don’t Cover Up Bad Smells- Fix Them!

Using air fresheners, candles, and scented oils only masks the foul odors from your central air conditioning system.

If you don’t address the cause, the air quality in your home will be compromised, your air conditioner can fail, or even your home can be catastrophically damaged. 

When To Call An AC Specialist

If you’re having unpleasant odors coming from your air conditioner and your troubleshooting has not fixed the issue, you should give our HVAC contractors a call.

Our team of experts can schedule a service visit, repair the HVAC unit, and get your home smelling like roses in no time.

Meet Your HVAC Expert

Lester Mclaughlin

HVAC systems are highly technical and often is the most misunderstood part of the house. From ductwork to heat pumps, I've been exposed to all sorts of issues facing homeowners. It really irks me when a homeowner is given bad advice like refilling freon vs fixing a leak in the system. I'm here to help our website readers with their heating and a/c problems.
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