Your air filter stops debris that comes in through your air intake before it reaches the throttle body or, in some cases, the carburetor. It can stop your engine from filling up with muck, dirt, and dust, but a dirty air filter restricts airflow to the engine, causing a rich air/fuel mixture. When your car runs rich, it burns more fuel and gives off more emissions. If you’ve ever looked at the air filter while changing it, you know just how much dirt it stops—and how dirty it gets. The filter keeps all that muck out of your engine, but a dirty one can clog up your engine’s airflow. Without sufficient airflow, your engine’s fuel air mixture runs rich with too much gasoline.