OEM Lennox Limit Switch — 18M84

The genuine OEM Lennox 18M84 limit switch is a temperature-activated safety control mounted on the heat exchanger or plenum of your Lennox furnace. It continuously monitors supply-air temperature and cuts power to the gas valve or burners if the furnace overheats, preventing heat exchanger damage and unsafe operation. On many systems it also signals the blower to run, ensuring heated air is removed from the cabinet.

A failing 18M84 often causes intermittent no-heat calls, a furnace that fires and then shuts down on high limit (short-cycling), the blower running continuously, or repeated lockouts. Nuisance trips are frequently the result of a weak or drifting limit switch, restricted airflow from a dirty filter, or a cracked bimetal element inside the switch.

Choosing the OEM Lennox 18M84 is important because limit switches are calibrated to a precise trip and reset temperature specific to your model. A substitute with the wrong setpoint can either trip prematurely — leaving you without heat — or fail to open when it should, defeating a critical safety function. This factory-matched replacement restores reliable overheat protection and proper blower operation for your Lennox equipment.