4 Potential Reasons Why Your Car Air Conditioner Smells Bad

In hot summer months, getting in your car and turning your air conditioner on to full blast is a sweet relief. The air conditioner allows you to have a comfortable drive, whether you’re just heading to work or settling in for a road trip.

This drive can quickly turn uncomfortable, however, if the air being blown into your face suffocates you with a bad smell instead of refreshing filtered air. The air can smell musty, like eggs, gas, sweet or anything in between. It’s easy to see that there’s something in your car that needs looking after.

Your car air conditioner hasn’t always had this smell. So, why does your car air conditioner smell now?

Why car air conditioner smells.

1. Bacteria and Mold

Mold is the most common reason for a bad smell from your air conditioner. In hot months, your air conditioner is used more often. The unit does not have sufficient time to dry properly.

The air conditioning unit in your car sucks air in from outside, filters away the heat and moisture from it and then pushes the cool air into your car. The air getting pushed into the car is dry, which means the moisture has to go somewhere.

The air gets pushed through the evaporator coils to be cooled down. As the hot air comes in contact with the cold coils, water condensates into pools that are then let out through the drain tube.

Overusing your AC means the water pools have less time to dry and the unit is always damp. Darkness and moisture are a breeding ground for bacteria which then turn to mold and mildew.

It is also possible that there is a problem with your drain tube. The water is then trapped in the AC unit. You are then faced with the same problem: moisture.

The mold usually builds up on the evaporator coils. Because the air filters through these coils, the smell of the mold is carried with the cold air through the dash vents and into the car.

Sometimes it is not overuse of the AC, but just that your car is getting old. Whatever the case, taking care of your AC is an important way to keep enjoying your drives.


Once you start to notice a smell, you wanna take care of it right away. You can use a homemade solution or get a commercial product. Kool-It and Wurth are two antiseptic products that can help, and they are available on Amazon at:

Alternatively, you can even use Lysol, which can be found in most supermarkets and drug stores. Products like Lysol and Wurth involve simply spraying them into the vents on the outside of the car. Make sure you set your AC to the option that pulls the air in from outside (and not recirculation). This way, the antiseptic can tackle the mold directly as it gets filtered through the evaporator coils.

Kool-It is a little more in-depth than the simple antiseptic sprays. To use it, you have to apply it directly to the evaporator coils. Take a look at this video to see one way you could do it yourself.


Fixing the smell is great, but you can also stop it from ever happening:

  • The best way to prevent mold is to use an anti microbial agent, such as Ultra-Fresh. This product kills bacteria and microorganisms before they ever turn to mold.
  • Another easy way to prevent mold is making sure the AC unit dries properly. You can do this by turning off your AC but keeping the fan running for a few minutes before turning off the car. Your AC unit then has a chance to fully dry. No moisture, no mold.
  • Take the AC off recirculation mode if you don’t need it. New air is better.
  • Do not turn the AC to full blast.

2. Dirty Air Filter

To keep dust and other tiny debris at bay, cars have an air filter. If this filter is dirty or clogged in some way, moisture can also get caught in this dirt. Ultimately, you face the mold problem all over again, except from a different source. If there is mold on the air filter, the smell of it still gets pulled in with the air.

Luckily, this problem also has a simple solution. Simply replace the air filter. It is relatively inexpensive, but will go a long way for helping your AC unit.

You can most likely find a new filter on the internet, depending on your car’s make and model. You can then change the filter yourself. Regular cleaning of the air filter can even extend its life.

3. Gas Leak

This reason is simple: if it smells like gas, then it is probably a gas leak. When you turn on your AC, the smell of the gas is mixed in with the outside air that your car is sucking in. The smell follows all the way through the system into your car.

Make sure to check:

  • The fuel pressure regulator
  • The fuel injector rings
  • The exhaust pipes
  • The muffler

You’ll either be able to see a leak in the first two or a hole in the last two.

You will definitely have to go to the mechanic for this one.

4. Antifreeze Leak

If there is a leak in the cooling system, the smell in your car will be sweet. What you are smelling in this case is ethylene glycol which is a toxic liquid used to regulate the temperature of the engine and prevent it from overheating. If the bad smell is sweet, take your car to the mechanic without delay.

The damage can be to

  • The heater core, pipe or hose
  • The coolant housing unit
  • The radiator

Not only is the liquid toxic to your health, but without it, your engine is vulnerable to damage that can be costly to repair. You’re better off fixing whichever part is causing the leak than replacing your car’s engine.

Alternatively, you’ll be able to tell there is most likely a leak in the cooling system if you turn on the heat and the air comes out cold. This is because there is no antifreeze left in the car so nothing is regulating its temperature. The engine isn’t being cooled off. Drive too long without antifreeze and your car could start smoking on the highway.

In Conclusion

In most cases, the reason your air conditioner smells when you turn it on will be the first two. These can easily be fixed without ever having to get your car checked. But if the cleaning out the evaporator coils and changing the filter does not get rid of the rotten smell, you should get your car checked.

Though the odor is unpleasant in all 4 cases, it does let you know that your car needs some TLC (tender love and care). It gives you the opportunity to fix the problem before it is too late. A small, smelly fix is better than a big expensive one.




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