Airfilters are important to an engine. Without an airfilter, dirt will enter into the engines manifold, and destroy an engine, or cause it not to run. Airfilters should be replaced when they are dirty. If the element inside the airfilter is black, its time to replace it. With a dirty airfilter the gas milage on a vehicle will decrease, and cause an engine not to run propperly. Airfilters are easy to replace.

This photo below is a example of a clean airfilter, and a dirty airfilter.