A Restriction Indicator is a device that is designed to measure and monitor the level of airflow through an air filter. As the air filter becomes clogged with dirt and debris over time, the airflow through the filter is reduced, causing a drop in air pressure upstream of the filter. The sensor or switch detects this drop in pressure and activates the warning light or gauge to alert the operator that the air filter requires cleaning or replacement.

Indicators help prevent under or over-servicing, loss of performance, and potential engine damage. They can be mounted directly on the air cleaner or may use remotely-mounted displays for operator convenience.