Whether you’re preparing your car for winter or simply doing regular maintenance, air filters are rarely at the top of the list. And when they are, it’s usually the air filter in the engine. However, with relatively few exceptions, cars also have cabin air filters. And yours probably needs to be changed.

What does your car cabin air filter do?

Cabin air filters, also called A/C air filters, do the same job that the engine air filter, Autoblog explains. Namely, keep incoming air clean. Only instead of filtering the air going into the engine, cabin air filters clean the air passengers breathe in. Dust, pollen, tire particulates, etc., it all gets trapped before it goes into your lungs.

However, as their alternative name suggests, cabin air filters don’t just clean up outside air. A/C air filters are also part of your car’s HVAC system. Whether you’re running the heater or the A/C, these filters remove airborne contaminants, Autoblog explains. That includes things like smoke from cigarettes and wildfires, The Drive reports.

Naturally, cabin air filters can’t remove every single contaminant. Some are better suited to removing particulates and odors, while others claim to remove viruses, The Drive and Roadshow report. It’s largely a matter of how large the contaminant is and what the filter is made out of. Though it’s worth pointing out that if you really want to sterilize your cabin air, you need a dedicated air purifier, which not every car has, Roadshow reports.

What happens if you don’t change your car’s cabin air filter?

As with an oil or fuel filter, a car’s cabin air filter eventually clogs up and can’t do its job properly. Luckily, there are several signs to watch out for.

One of the most obvious is poor airflow, Autoblog reports. If the A/C air filter is clogged, even if the fan’s working, the cool air will struggle to come in. Ditto the warm air if you’re trying to run the heater. The HVAC system may also make more noise, especially from the air vents, Car and Driver reports.

Another sign of a failing cabin air filter is a musty odor that shows up when you try to run the fan, Autoblog reports. Mold and mildew can build up in your HVAC system, and a clogged filter is often the cause.

There are a few more subtle signs that your A/C air filter is in need of replacement. Your car’s windows won’t clear as quickly if you run the defogger, Carfax reports. And a clogging filter also lets more condensation build up on the windshield.

How do you change it?

Your cabin air filter’s replacement interval is based on a number of factors. Besides your specific make and model, it also depends on where and how much you drive your car, Cars.com reports. A general rule of thumb, though, is about every 12,000-15,000 miles, or 1-2 years, NAPA and Cars.com report. And it’s something that’s fairly easy to do by yourself.

Cabin air filters are typically located behind or around the glove box, Autoblog reports. Simply unscrew the glove box and you should be able to access the filter. The filter itself is very similar to the engine air filter, just larger. Once you have it exposed, just unscrew or release it, and put in a new filter.

Speaking of new filters, you can always upgrade your existing A/C air filter. If you’re really concerned about particulates, HEPA-rated filters can trap particles as small as 0.3 microns, AAA reports. And some come with activated charcoal or other add-ons for better odor or gas removal. Though naturally, such filters cost more.

It’s also worth pointing out that not every car on the road has a cabin air filter. The vast majority do—O’Reilly Auto Parts claims roughly 80% of 2000-and-later models have these filters. But my 1999 NB Miata, for example, doesn’t have one. So, if your car’s an older model, and you can’t find its A/C air filter, know that it may not have one.

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