OEM Lennox Pressure Switch — 51M87

The genuine OEM Lennox 51M87 pressure switch is a combustion safety control that confirms the draft inducer is moving air before the furnace allows ignition. It senses the negative pressure created by the inducer through a small hose; when adequate draft is proven, the switch closes and permits the ignition sequence to continue. This part is calibrated to the exact set point your Lennox furnace requires so it neither trips prematurely nor allows firing without proper venting.

When the 51M87 fails or drifts out of calibration, the furnace typically locks out on a pressure-switch fault and produces no heat, or it short-cycles as the switch opens and closes erratically. Blocked vents, a weak inducer, or a cracked hose can mimic these symptoms, but a stuck-open or stuck-closed switch is a frequent root cause.

OEM is critical here because the switch's trip point is matched to your specific Lennox model's venting design. An aftermarket switch with the wrong set point can allow the burners to fire without adequate draft — a serious safety hazard — or cause constant nuisance lockouts. Install the genuine Lennox 51M87 to restore accurate, reliable draft proving and safe furnace operation.