The cabin air filter is located underneath the dashboard of your vehicle. The purpose of the cabin air filter is to keep the allergens and dust out of the car's HVAC system. In order for your HVAC system to stay clean, the cabin air filter has to be changed on a regular basis. Most vehicles need to have their cabin air filter changed once every 20,000 to 30,000 miles. However, it is a good idea to ask a mechanic or check the owner's manual.