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The Lennox 21W00 (OEM part 101021-01) is a genuine OEM two-stage pressure switch rated at .60/1.00 inches of water column, used in Lennox two-stage furnaces to confirm that the draft inducer is generating adequate negative pressure before the ignition sequence is allowed to proceed. Symptoms of failure include a furnace that won't light, repeated lockouts, or fault codes indicating a pressure switch open condition during either stage of operation. As a direct OEM Lennox replacement, this part delivers the precise switching thresholds needed for safe, reliable two-stage furnace performance.