This switch detects a pressure differential across the air filter element. It is mounted in the air intake system between the filter housing and the engine intake. When the filter becomes loaded with debris, the rising restriction inside the intake tract causes the switch to close, completing a circuit that illuminates a warning lamp in the cab. A failed switch will either fail to trigger the warning when the filter is clogged or trigger a false warning with a clean filter. The switch is calibrated to the original spring and diaphragm assembly, so it activates at the correct restriction threshold for the engine's air demand. On replacement, ensure the sealing O-ring is seated properly in the housing groove to prevent unfiltered air from entering the intake.