Updated April 09, 2019

Your cars’ air conditioning unit is a vitally important system in terms of making your driving experience a pleasurable one, especially as the temperatures outside rise.

There is nothing better than escaping the humidity by placing the AC on full blast in the summertime. Unfortunately, overuse and misuse of the air conditioning unit in your car (and sometimes just bad luck), can cause your cars’ AC to begin to emit some terrible smells.

If you’ve ever had this happen in your car or been riding in a vehicle with the problem, you know exactly what we're talking about. You get into a hot car and instantly reach to turn up the AC, only to be hit in the face with an unpleasant (and unwanted) smell.

This smell has been described in many ways – musty, rotten eggs, dirty socks, fishy, burnt plastic, gas, to list a few – and can even be accompanied by the feeling of being short of breath, dizziness, or headaches. No one wants that!

Let’s talk about why these smells occur and how we can prevent them.

What Causes the Car AC to Smell Bad?

In short: moisture.

Your vehicle’s air conditioning system does not actually create cold air; it takes heat and moisture out of the air that is outside of your car, which in turn, makes it feel much cooler than the air around you.

The coils in the AC unit work to condense the heat and moisture in the air, taking out the humidity before pushing the cooled air into your car.

As these coils are colder than the air around them, hot air flowing into the unit can create moisture pockets that surround the condensing unit in your AC system; which create small pools of water that are drained outside of the car via the drain line.

However, if moisture becomes stuck or for whatever reason cannot find its way out of the AC unit, by drainage or evaporation, the water will pool inside the unit and around the coils creating moisture inside of the AC unit.

The culprit: mold.

If your drain line is damaged or you frequently use your AC system, the unit is likely to stay damp all the time, which creates a breeding ground for mold and bacteria.

The air that passes through the evaporator and the units’ mold build-up will carry the foul smell inside of your car through your dash vents.

Mold is the most common culprit for causing raunchy smells to come out of your cars’ AC.

As a vehicle gets older, bacteria, mold, and other unwanted micro-organisms can start to grow. If the AC unit is not often used, or used much too often, this can also increase the growth of these microbes inside your car’s AC system and ventilation system as a whole.

Odor-causing mold loves to reside in your cars’ AC unit because it is always moist and dark – ingredients for a happy home for mold.

How to Prevent Musty, Mildew Smells from Car AC

Your first task of action should be to locate the drain tube in your vehicle and ensure that it is functioning properly. You might want to bring your vehicle into a trusted dealership or repair shop to prevent causing further damages to your vehicle.

If the drain tube appears to be in proper working condition, you can rule this problem out and look at killing off the mold currently within your cars’ AC system in addition to preventing new mold from forming in the future.

Here are some ways you can do so…

1. Turn off the AC 2-5 minutes before you shut off your vehicle but leave the fan on. This will allow the entire AC unit to dry much more thoroughly every time you use it.
2. Clean or replace your AC filters. Dirty air filters reduce airflow through your AC and help moisture remain inside the unit. Be sure to replace the AC filters as often as recommended by the manufacturer of your vehicle.
3. Spray an antiseptic into the exterior air vents (where your car pulls air from outside into the car from). Be sure your AC is set to the setting that pulls air from outside in, so that the spray is pulled into the ventilation system and can directly work at killing of mold and bacteria.

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Car ac smells bad? Try changing your air filter as it can collect debris that can introduce mold or act as an additional food source for mold.

My Car AC Still Smells Bad, Now What?

If you checked on your drain tube and tried to clear out mold yourself with no prevail – all is not lost.

You don’t have to replace your car, but you MIGHT have to replace the AC unit in it.

Bring your car into a trusted dealership or repair shop to have your AC unit assessed as soon as possible.

How Ultra-Fresh Can Help Prevent Car AC Smells, Like Mildew

By incorporating an antimicrobial agent into the foam, filters, rubber and adhesive components of the Car's AC system, mold isn't able to begin growing in the first place.

Wondering what an antimicrobial agent is? Read more to find out.

Here are examples of foam, used in car AC systems, after testing via the ASTM G21 antifungal method.

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Antimicrobial treated foam, used in car AC systems, inhibits mold growth and prevents musty odors

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Untreated foam, used in car AC systems, will grow mold and can contribute to musty odors