OEM Lennox Furnace Limit Switch — 66J78

The genuine OEM Lennox 66J78 is a high-limit safety switch that monitors the temperature inside the heat exchanger area of compatible Lennox furnaces. If the furnace overheats — typically from restricted airflow — the 66J78 opens the circuit to shut off the burners, protecting the heat exchanger and preventing a hazardous condition. It resets automatically once temperatures return to a safe range.

When this limit switch fails or the furnace experiences airflow problems, you may see the burners short-cycling, the furnace shutting down mid-cycle, or the blower running continuously while producing little heat. A switch that has tripped permanently or drifted out of calibration can lock the furnace out entirely, leaving your home cold even though the rest of the system appears healthy.

Using a genuine OEM Lennox part is essential for any safety-critical control. The 66J78 is calibrated to the precise trip and reset temperatures your Lennox furnace requires, so it protects the equipment without nuisance shutdowns. Aftermarket switches with incorrect temperature ratings can either fail to protect the heat exchanger or trip prematurely. This factory-matched limit switch restores dependable, safe operation with an exact fit.