OEM Lennox Pressure Switch — 57M66

The genuine OEM Lennox 57M66 pressure switch is a furnace safety device that verifies proper combustion venting before ignition. Wired to your furnace control board, it senses the negative pressure created by the draft inducer motor and closes its contacts to prove that flue gases are being drawn out of the heat exchanger. Only then will the sequence advance to open the gas valve and light the burners.

Common failure symptoms include the furnace failing to ignite, cycling on and off without producing heat, or a diagnostic code indicating an open pressure switch. A worn diaphragm, a cracked or clogged sensing hose, condensate blockage, or a restricted vent can all keep the 57M66 from closing, causing the furnace to lock out as a protective measure.

OEM matters here because pressure switches are calibrated to a precise trip point in inches of water column that is specific to each Lennox furnace model. Installing a mismatched aftermarket switch can lead to unsafe firing or persistent nuisance faults. The 57M66 is built to exact Lennox factory specifications for correct calibration, a proper physical fit, and dependable long-term safety.