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The Lennox 73W35, also referenced as OEM part number 102737-09, is a genuine 2-stage pressure switch rated at .55/.25 inches of water column for use in Lennox gas furnaces requiring dual-stage draft pressure verification prior to ignition. This switch ensures that the venting system and inducer motor are functioning correctly, locking out the furnace if proper pressure differentials are not met and protecting against hazardous combustion gas conditions. Common failure symptoms include furnace lockout, pressure switch fault codes, or continuous inducer operation without a successful ignition cycle when the 73W35 is faulty.