How to get rid of or reduce a bad smell from A/C vents in a car

December 16, 2019

Where does the odor from the A/C vents come from? Let's look at the air flow in the HVAC system of a car. Fresh air travels through the cowl panel, then through the cabin filter, then through the A/C evaporator and the HVAC unit and then comes from the vents; see the diagram.

In-car HVAC system
A/C system and HVAC unit inside the car.

Accumulation of moisture and debris in any of these areas might cause mold and bacteria to grow, resulting in a bad odor from the A/C vents.

Leaves and other debris can slide through the cowl panel and get onto the cabin filter, see photos below. With the help of moisture, they begin to decompose, causing musty or moldy smell inside. Small leaves can get around a cabin filter and clog the A/C drain tube. This will also cause accumulation of moisture inside the HVAC unit. The HVAC unit is buried behind the dash and is difficult to access.

The fresh air intake is close to the engine bay, which means some of the smells from the engine bay (e.g. burnt oil smell) also penetrate into the cabin. A burnt oil smell is often caused by oil leaks.

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Another reason for an unpleasant odor can be caused by small critters that store seeds, nuts and other supplies in the engine bay. It's not always possible to completely get rid of a bad smell, but there ways to reduce it. The key is finding the source of the smell. Here are a few things you can do:

Keep the A/C drain in your car clean

A/C drain tube
A/C drain tube under the car.

If the A/C drain tube is partially blocked, the air inside the HVAC unit remains moist. During the next oil change, ask your mechanic to check if the A/C drain tube is not clogged up. If it is clogged up, a mechanic might suggest cleaning it with a vacuum or compressed air. The A/C drain tube is located on the passenger side of the firewall or extends to an area under the car, like in this photo. You can find it following water that drips under the car when the A/C is running.

One of the symptoms of a clogged-up A/C drain is when you can hear water splashing inside the HVAC unit. In some cars it's noticeable when turning, when water flowing towards the blower motor makes the splashing noise. Usually it happens when turning left, because in most left-hand-drive cars, the blower motor is located on the right side.

Clean the leaves and other debris from the cowl panel

Leaves on the cowl panel
Leaves and other debris collecting under the windshield will cause rust and moldy smell inside the car.

Clean the debris from the cowl panel (below the wipers), as fresh air flows into the cabin through this area. Often a damp moldy smell comes from wet leaves and other debris collected there. If possible try to see through the cowl panel cover; some debris might be visible inside.

The cowl panel covers the water channel that runs parallel to the windshield. It's the channel where the wiper motor is installed in most cars. The channel has drains on both sides. Leaves and twigs falling through the cowl panel can clog up those drains. This can also cause accumulation of moisture and a damp moldy smell from the vents. If the cowl channel drains are completely clogged up, water can leak inside the car during rain.

Change your cabin filter regularly

New versus dirty cabin air filter
A dirty cabin filter can also cause bad odor from the vents.

The cabin filter in your car is similar to a furnace filter in your house. It filters fresh air before it enters the HVAC system. Like any other filter, over time, a cabin filter clogs up, see the photo. Signs of a dirty cabin air filter include reduced air flow and a bad smell from the vents. Dust, pollen and other contaminants trapped in the cabin filter can also cause problems if you or your passengers have allergies.
If your car is parked under trees or if you often drive on unpaved roads, the cabin filter clogs up faster.

If you want a cabin air filter that better absorbs smells, look for a high-performance charcoal filter. Replacing a cabin filter more often will also help in reducing odors. Replacing a cabin filter in a repair shop costs from $25 to $115 (labor), plus the part ($15-$45). Scroll down to see how to replace the cabin filter in the Toyota Corolla.

When parking the car, turn the HVAC system to the "fresh air" mode

Fresh/Recirculation mode
Fresh/Recirculation mode

This will allow some fresh air inside the cabin. In the Recirculation mode, the fresh air flap is closed, trapping the air inside the cabin.

HVAC deodorizing service

A/C treatment spray
A/C treatment spray

Many dealers and auto repair shops offer an HVAC service, where a mechanic sprays a disinfectant into the HVAC system to kill off the bacteria and fungi on the A/C evaporator and other areas. For example, in the photo we used the Air Conditioner Treatment from Wynn's (check for precautions before using the product).

Of course it's important to use only the approved product and follow the instructions, as breathing strong chemicals is also not very healthy. Some products require running the HVAC system for a certain time with open windows to let the chemicals out before the car can be driven.

Keep the interior clean

Freshly-cleaned car interior
Freshly-cleaned car interior

A dirty interior will also cause a bad odor that seems to come from the vents. We have seen cases where a rotten fruit or leftovers that fell under the seat caused the whole interior to smell bad. Check all hidden areas, especially under seats. Even a spilled coffee over time can cause a bad smell. Read: how to clean a car interior.

If you don't have time to clean the interior yourself, check with a local detailing shop or a dealer; their prices are often reasonable. Many professional auto detailing shops offer not only detailing, but ozone treatment that helps get rid of bad odors. The ozone also kills bacteria and fungi. We tried the ozone treatment on several cars and it does help.

Look for signs of moisture under the carpet

Moisture under the carpet
Moisture under the carpet

Moisture under the carpet can also cause a bad moldy smell inside. If you do find moisture under the carpet, it needs to be dried up. In this car in the photo, moisture under the carpet caused not only a bad smell, but corrosion in the wires. We know cases where the whole carpet in a car had to be replaced to get rid of a bad smell.

Another common source of the moldy damp smell inside the car is moisture inside the trunk. Often, you can find water that has accumulated inside the spare tire well.

How to replace a cabin filter (2009-2013 Toyota Corolla)

Changing cabin air filter step one

In some cars, replacing the cabin filter is easy; in others it may require more labor. Typically, the cabin filter is installed behind the glove box. For many cars, you can find the instructions on how to replace the cabin filter in the owners manual. The owner's manual for this Toyota Corolla includes illustrations.

Changing cabin air filter step two

As per the owner's manual, the first step is to turn the engine OFF; of course, we don't need it running. The next step, open the glove box, locate the damper and slide it off. Now, the manual says push in each side of the glove box to disconnect the claws. To pass these claws over the edges, you need to squeeze the glove box gently from each side.

Changing cabin air filter step four

The cabin filter in this Toyota is behind the black plastic cover. Pull the cover out and you can see the filter.

Changing cabin air filter step five

Pull the old filter out carefully, as it might be filled with leaves and other debris. If the leaves fall through they may cause a flapping or squeaking noise when the blower motor is running.

Changing cabin air filter step eight

Insert the new filter back the same way. This manual says The "↑UP" marks shown on the filter should be pointing up. Slide the filter back carefully; check if the cover closes properly. All we need now is to install everything back in reverse order.

Changing cabin air filter step ten

Again, push in each side to make the claws go back in.

Changing cabin air filter step eleven

Lastly, push the damper back into its place. Done. A cabin air filter is recommended to be replaced every 12-24 months or 15,000-25,000 miles.




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