Symptoms of a dirty air filter vary but will often times include a noticeable decrease in gas mileage. Other symptoms are potential ignition problems cause by fouled spark plugs. A dirty air filter prevents the necessary volume of clean air from reaching the engine which affects the emission control systems of the
car; reducing air flow and causing a too rich air-fuel mixture which can foul the spark plugs. Fouled spark plugs can create an engine miss, rough idle and even starting problems. In addition, a too rich fuel mixture increases engine deposits which may even cause the Service Engine Soon light to come on.
A brand new air filter will generally appear white to off white. A visual inspection of an air filter may reveal that the air filter has become darkened or full of debris. A visual inspection can be done by examining the air filter in bright light. However, an air filter can become clogged with dust and even pollen that may not be easily seen. Therefore, most car manufacturers recommend replacement every 12,000 to 15,000 miles regardless of whether or not the air filter appears to be clean. Cars driven in dusty or frequent stop and go conditions will requires a more frequent check and replacement of the air filter.