3 Reasons Why Your Car AC Smells Musty (with Fixes)

Although it’s unpleasant – and even a little embarrassing-  a musty smell from your car’s aircon is not dangerous. 

Here we’ll look at the causes and fixes to get your car cabin smelling fresh and clean again.

Why A Cars Aircon Systems Smell Musty

This musty smell is caused by mildew and mold forming in your car’s aircon system. Cold air is forced past mold or mildew deposits and into the cabin when the AC is turned on.

Where Is The Musty Car AC Smell Coming From? How To Fix It

1. Evaporator Drain Hole Clogged

When your AC is on, you should notice a small pool of water on the ground underneath the engine. This is normal and is the evaporation from the normal workings of the aircon system.

The drain hole where the water escapes can become clogged quickly by leaves or pine needles. When this happens, the water collects in the evaporator and stagnates.

Stagnant water is prone to bacteria and mold. Depending on your car’s make, you may have noticed wet patches on the floor on either the passenger or driver’s side. This is because the evaporator is backed up with water that can’t escape and is overflowing.

As you turn a corner, you might hear a sloshing noise. This can be coupled with a really bad smell too.

The fix.

The drain hole is located under the hood poking through the firewall. It’ll be black and look like the end of a hose. If the aircon is on and the end is dry, this indicates it’s blocked.

A quick DIY fix is to unravel a coat hanger and poke one end through the hole a few inches. Wiggle it around, and if it works, you should see and smell pungent water escaping. 

If you still notice that your car smells musty, then look at the second reason below.

2. Clogged Air Vents and Intake Manifold

The air vents and hoses can also be vulnerable to mold and mildew forming.

Leaves and other debris can get sucked into the intake manifold of a car – usually, they’d be captured by the air and cabin filter if appropriately fitted- and once inside, they have nowhere to go apart from through the air vents. 

Leaves in air vents cause a musty car smell
Leaves can find their way into a car’s air vents.

Occasionally you’ll see part of a leaf blow into the cabin when you put the blower on, but most debris will be too big for this to happen. 

The result is that the debris will break down to mold inside the hoses. This happens more when the air is humid as decaying matter and humidity are ideal conditions for mold to grow and thrive.

The fix.

Get under the hood to see if you’ve any leaves blocking the air intake vents. If so, remove them right away.

You may not be able to get to them all by hand so try a car vac. If this doesn’t work, you may have to remove some screws and take off the plastic housing to get to the more awkward ones.

This won’t get rid of the smell, but it will stop it from recurring. 

To get rid of the smell, you can either. 

Buy stronger air fresheners – I know, not a long-term fix- but the bacteria is still getting into the cabin. 

An air freshener will mask, not solve the problem.

Or disinfect and clean the air hose and vent circuit. There are many ways to clean this, but the easiest is to use a disinfectant canister can that you activate inside a car’s cabin with the aircon on.

Car disinfectant canister
A disinfectant canister will clear musty smells

It works by recirculating the canister’s contents through the air vents to the evaporator and back again and cleans the whole system as it does so. It takes 15 minutes to do its magic and requires no special tools or knowledge.

3. Cabin Filter Clogged

The cabin filter’s job is to stop debris from entering the cabin. They usually work fine if replaced in line with the manufacturer’s guidelines – usually every 30,000 miles.

Clean cabin filters to clear musty smell
Changing over cabin filters

If they aren’t replaced or are poorly fitted, they can absorb small amounts of moisture and dampen the filter material causing it to rot.


If you have looked at the other two reasons but found they weren’t the cause of the smell, then the cabin filter is a likely cause. Replace it.

When Is a Car Most Likely To Smell Musty?

All car aircon systems produce condensation; it’s a by-product of creating the gases used to cool down the air we feel through the vents.

Newer cars are less likely to smell moldy as the aircon parts are more efficient when new. Also, the amount of mold needed to produce the musty smell can take a few years to get to the stage of smelling bad.

A car’s aircon is most likely to smell like this when the car has been parked for some time or when the aircon system hasn’t been used recently. 

A common time to smell this is in the spring when we turn on the aircon for the first time in a few months. The mold has had time to grow undisturbed and wouldn’t have been expelled from the system as it would be in the summer months.

What To Do If The Smell Remains.

It’s time to get your car booked in for an HVAC service. They will thoroughly inspect your car’s aircon system, which may involve the draining of the Freon gas from the system.

Professional HVAC lean
Professional HVAC cleaning

A car shop must do this as this gas is toxic and needs to be disposed of or recycled correctly. 

How to Prevent Musty, Mildew Smells From Your Car AC In The Future.

Once your aircon has been repaired, there are certain things you can do to ensure the smell doesn’t come back.

  • Run your aircon at least once a week – even in winter – as this gives less chance of bacteria and mold taking hold in the evaporator.
  • Once a month – with the aircon running – check underneath the car to see if a puddle is forming. There should be, as this indicates the evaporation hose is clear.
  • Replace the cabin filter in line with the car manufacturer’s guidelines
  • As a precaution/preventative measure, use an in-car disinfection canister to clear mold before it gets a chance to establish itself.

To Recap

  • The musty smell coming from your aircon is almost always caused by a buildup of mold within the evaporator of your AC circuit.
  • Sometimes, the smell of leaves or pin needles decaying in the air intake manifold or hoses can enter the cabin – especially if the cabin filter is clogged.
  • The cabin filter should be changed often, and if moisture is leaking onto it, it can grow bacteria in the cabin.
  • Most of the fixes are generally cheap and can be done by a basic car DIY’er. 
  • Once you’ve ruled out the common problems and fixes, it’s time to get a professional to look at your AC. Even at this point, though, the issue is likely to be a cleaning or disinfecting issue. 
  • Professional HVAC shops have access to better cleaning systems and products and should fix the issue quickly. Only rarely do AC parts need to be replaced.

Related Articles:

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Another common car aircon smell is vinegar! If you think your car smells more like this, check out our guide to Why My AC Smells Like Vinegar.

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