The cabin air filter, a feature found on most late-model vehicles, cleans the air that comes into the interior through the heating, ventilation and air conditioning system. It catches dust, pollen and other airborne material that can make riding in a car unpleasant, particularly if you have allergies or other respiratory problems.
Recommendations on when it should be replaced vary by manufacturer — some say every 12,000 or 15,000 miles, others longer — and how often can depend on how much you drive and where. Check the maintenance schedule in your owner's manual. If you drive in heavy traffic in an urban area that has poor air quality, you could need to replace the filter annually or even more often. However, that also could be true in a desert climate where there is a lot of dust.
Some signs that you need a new cabin air filter are reduced air flow through your HVAC system, such as when you crank up the fan too high and you get more noise than results. Another is persistent bad odors. Even if you don't have these warnings, you should have the filter checked at least once a year, and you may be able to do that yourself.
Many cabin air filters are located behind the glove box and are easily accessible by freeing the glove box from its fasteners (instructions should be in the owner's manual). Others are located under the dashboard and may not be easy to reach, or under the hood where fresh air enters the HVAC system. Some of these filters are expensive, as in $50 or more at dealerships, so you could save money by buying a replacement at a parts store and doing it yourself.
If a dealership service department or repair shop recommends you get a new cabin air filter, ask to see the current one. Depending on how long the filter has been in service, you might be shocked at what you see: leaves, twigs, insects, soot and grime that literally cover the entire surface that comes in contact with incoming air. You'll know it's time for a new cabin air filter.