Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
When your car's air conditioning system smells bad, it's usually a musty and moldy odor. Often, the culprits are:
- a cabin air filter in need of replacement
- a humid evaporator, or
- an AC evaporator clogged with mold or fungi.
During the normal operation of your car's ventilation, heating, and air conditioning (AC) systems, the cabin air filter helps trap moisture, dirt, soot, pollen and other foreign particles to provide clean air to the system. Eventually, trapped particles and mold eventually clog the filter, reducing AC efficiency and causing the bad odor you smell when the AC operates.
Also, during AC operation, the evaporator drains water from the moisture it removes from the air. Over time, the evaporator's drain may clog and water becomes trapped. This provides a damp environment good for mold to grow around the evaporator and ductwork.
The next sections show you how to inspect your car's cabin air filter and evaporator and how to do a simple treatment to get rid of your car's bad AC smells.
In This Article
- Check the Cabin Air Filter
- Check the AC Evaporator
- Fighting the Source of Your Car Air Conditioner Smell
- Keeping Your AC System Odor Free
1. Check the Cabin Air Filter
The cabin air filter is designed to trap pollutants like pollen, dirt, and soot that find their way through the AC, heating and defrost systems.
Most newer vehicles come with a cabin air filter that is usually replaced once a year. Check your car owner's manual or vehicle repair manual for the filter location and service schedule.
Finding the Cabin Filter
Usually, you'll find this filter:
- behind the glove compartment,
- under the dashboard,
- under the hood,
- or in the engine compartment in a case between the blower motor and evaporator core.
Remove the filter from its case and examine the element. If it is too dirty or clogged, the filter is not allowing sufficient air to pass through, affecting HAVC system operation and probably fouling the air that circulates inside the cabin.
Types of Air Filters
There are different types of air filters:
- particle-trapping with a charcoal layer
- electrostatically charged to trap smaller particles
- capacity for large particles
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The charcoal layer helps to suck in odor-causing particles. Other types of filters may even prevent the growth of mold, algae, yeast, mildew, and other bacteria. Check your car owner's manual or repair manual for a suitable filter for your application.
2. Check the AC Evaporator
If your cabin air filter is good or newly replaced, but the AC smells continue, then check the evaporator.
The evaporator helps to drain the humidity removed from the air by the AC system. Common evaporator failures include a clogged drain or leaks that may cause water to accumulate under the carpet.
How to Check for a Clogged Evaporator
On a hot summer day, you can do a simple test to check if your evaporator is clogged:
- Park your car in a safe place and engage the parking brake.
- Start the engine and let it idle.
- Turn on the AC and set it to MAX.
Wait for a few minutes.
- Water should start dripping under your vehicle as the evaporator drains the accumulation of water.
- If you don't see water dripping under the vehicle, this could be a sign of a clogged-up evaporator drain—and possibly the reason for the bad smell when running your AC.
You can further inspect the evaporator's drain tube on your particular model. Consult your vehicle repair manual.
How to Check and Unclog the Evaporator
- Jack up the vehicle and safely support it on jack stands.
- Chock the rear wheels.
- Find the evaporator case drain tube.
- Place a drain pan under the drain tube.
- Disconnect the tube from the case to drain it.
If no water comes out of the case, unclog the drain hole and tube.
- Use a rod to carefully unclog the case hole and tube. Be especially careful when working on the tube to prevent damage. In some cases, you may need to use low-pressure air to clear the tube.
- If water is still trapped inside the case after trying to unclog the unit, you'll need to have the evaporator passages cleared, or have the unit replaced.
3. Fighting the Source of Your Car Air Conditioner Smell
Unplugging the drain and replacing your car's cabin air filter may be part of the solution to the offensive odors you smell every time you turn on the air conditioner.
However, AC operation produces a humid environment around the evaporator that is conducive to the growth of mold, mildew and other fungi. Such growths will cause a musty smell, especially when the outside temperature starts to rise.
The next step is to use an anti-mildew, disinfectant spray to fight the source of the foul odor coming out of the AC vents.
How to Kill Fungi in the AC System
- From outside the vehicle, remove debris from the air intake at the base of the windshield.
- Start the engine and let it idle.
- Set the transmission to Park (automatic) or Neutral (manual).
- Turn the blower on, but don't turn on the AC.
- Set the ventilation system to let outside air circulate through the cabin of the vehicle.
- Roll down the windows and allow outside air to circulate through.
- Let the blower run for about 10 minutes to help the evaporator core's surface dry out, if necessary.
- Then spray disinfectant through the air intake vents on the windshield cowling. You may feel with your hand where the air is being introduced into the car.
- Spray the disinfectant for 20 to 30 seconds at a time, for about 10 minutes, to allow the interior of the air ducts to soak in the disinfectant.
- Turn the blower and engine off.
- Allow the disinfectant to dry for a few minutes and repeat steps 4 through 9 as necessary.
The following video shows you how to apply disinfectant into the AC vent system of your car to eliminate foul odors.
How to Apply Disinfectant to Your Car AC Ventilation System
4. Keeping Your AC System Odor-Free
Often, you can easily remove offensive odors from the car's AC system yourself.
However, if bad odors continue, you may have a mildew or mold problem around the carpeting and other areas of the passenger compartment. Usually, this comes from an evaporator leaking water under the vehicle's carpet, around the passenger's footwell.
In this case, you'll need to have the AC system and interior of the vehicle serviced at a shop or dealership.
Prevent Foul Odors From Happening in the First Place:
- Remove debris from the windshield cowling before you begin using the AC system during the summer months.
- Once a year, check the evaporator drain for signs of blockage, and, if necessary, clear the passage.
- Replace the cabin air filter once a year. Consult your car owner's manual.
- On a hot, dry day, turn the AC system off for about five minutes before shutting off the engine.
- Set the fan to circulate outside air into the cabin and run the blower for five or ten minutes before shutting off the engine. This will help eliminate humidity from around the evaporator.
- If you don't use the AC system in your car that often, run it a couple of times during the summer, following steps 4 and 5, to prevent the growth of fungi.
Repeat steps 4 and 5 throughout the summer to prevent moisture buildup around the evaporator.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Dan Ferrell