The cabin air filter 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.
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.
Depending on how long your filter has been in your vehicle and the conditions under which you drive, 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.