OEM Lennox Furnace Pressure Switch — 34M77

The genuine OEM Lennox 34M77 pressure switch is a critical safety component that confirms proper venting and combustion airflow before your furnace's ignition sequence begins. It monitors the negative pressure created by the inducer motor and closes only when draft is adequate, protecting your home from unsafe operation and flue gas backup. This factory-specified switch is calibrated to the exact set point required by your Lennox system.

Common failure symptoms: A failing 34M77 often prevents the furnace from firing, causes the unit to lock out on a pressure fault, produces short-cycling, or triggers repeated ignition attempts followed by a hard shutdown. Blockages, a stuck diaphragm, or a switch that no longer closes reliably are typical causes.

Why OEM matters: Lennox pressure switches are calibrated to specific pressure thresholds that vary by furnace model and venting configuration. Installing a generic switch with the wrong set point can cause nuisance lockouts or, worse, allow the unit to run when venting is unsafe. The 34M77 is the exact part engineered for your equipment, ensuring reliable ignition and rated safety performance.