OEM Lennox Furnace Limit Switch — 73K21

The genuine OEM Lennox 73K21 is a furnace high-limit safety switch designed to monitor heat exchanger temperature and protect your Lennox equipment from dangerous overheating. When the air temperature around the heat exchanger exceeds the switch's rated trip point, the 73K21 interrupts power to the burners or gas valve, shutting down the heating cycle until conditions return to safe levels. This is a critical safety component on many Lennox gas furnaces and air handlers.

Common failure symptoms: A failed or nuisance-tripping limit switch often causes short cycling (the furnace repeatedly firing and shutting off), no heat, the blower running continuously, or the furnace locking out on a limit fault. Over time, heat and thermal cycling can weld the contacts open or drift the trip temperature out of spec.

Why OEM matters: The 73K21 is calibrated to a specific trip and reset temperature for your Lennox model. Using a generic or mis-rated substitute can trip too early, fail to trip during a genuine overheat, or fit improperly against the heat exchanger. Installing the factory-specified 73K21 ensures correct temperature calibration, proper mounting, and safe, reliable furnace operation.