The air filter box stops dust and dirt from entering the engine, which can mix with oil and damage the engine. A dirty air filter box can affect fuel economy and damage the catalytic converter. It should be checked regularly and replaced every 30,000 miles or more often in dusty conditions. Ensure the replacement fits properly. To check its effectiveness, remove the air filter box assembly and replace it if the idle speed increases. For carbureted engines, remove the air filter box wing nuts, cover, and element, clean the housing, inspect the crankcase breather filter, and replace if needed. For fuel-injected engines, remove the air filter box cover and filter element, clean the housing, and reinstall.