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The Lennox 51W91 pressure switch is a combustion-airflow safety control that confirms proper venting in your Lennox furnace prior to ignition. When the inducer motor starts, it produces a negative pressure that the 51W91 detects through its sensing port. The switch closes only when sufficient draft is present, allowing the ignition sequence to continue and preventing operation when the flue or vent is obstructed.
Common failure symptoms: If the 51W91 fails, you'll often see the inducer spin up while the burners never fire, along with a pressure-switch error on the diagnostic board. Symptoms include short-cycling, intermittent no-heat calls, and repeated lockouts. A stuck diaphragm, blocked or cracked tubing, or a switch that has drifted out of calibration are typical causes.
Why OEM matters: The 51W91 is factory-set to the precise pressure specification your Lennox furnace demands. Aftermarket switches with incorrect set points may refuse to close or, dangerously, close when they shouldn't — bypassing a genuine venting fault. Installing the authentic OEM 51W91 ensures accurate calibration, proper fit, and complete compatibility with your Lennox equipment.