OEM Lennox Flame Sensor — 20J91

The genuine OEM Lennox 20J91 flame sensor is the safety device that confirms your burners have actually lit during each heating cycle. Using flame rectification, the sensor generates a small microamp current when it detects the presence of flame and signals the control board to keep the gas valve open. Without a valid flame signal, the board closes the valve within seconds to prevent unburned gas from accumulating.

Common failure symptoms: A dirty or worn 20J91 is one of the most frequent causes of a furnace that lights briefly then shuts off (short cycling), repeated ignition retries and hard lockouts, or intermittent no-heat calls. Oxidation and carbon buildup on the sensor rod weaken the flame signal over time.

Why OEM matters: The rod length, mounting geometry, and alloy of the 20J91 are engineered for correct flame immersion in your specific Lennox burner assembly. A mismatched aftermarket sensor can sit in the wrong part of the flame, produce an unstable microamp reading, or corrode prematurely. The factory OEM part restores a strong, stable flame signal and dependable ignition.