OEM Lennox Flame Sensor — 21L99

The genuine OEM Lennox 21L99 flame sensor is a critical safety component in your gas furnace. It uses flame rectification to confirm that the burners have actually ignited, signaling the control board to keep the gas valve open. When a valid flame is detected, your furnace runs normally; when it is not, the board shuts off the gas to prevent a dangerous unburned-fuel buildup.

Common failure symptoms: A failing or dirty 21L99 sensor is one of the most frequent causes of short-cycling. You may notice the furnace lighting for a few seconds and then shutting down, repeated ignition attempts, no heat despite the blower running, or a control board flashing a flame-sense or lockout fault code. Oxidation and carbon buildup on the sensor rod reduce the microamp signal until the board no longer trusts it.

Why OEM matters: Lennox designs the 21L99 to specific rod length, alloy, and mounting geometry so it produces the exact flame-rectification current the Lennox control board expects. Generic sensors often sit at the wrong depth in the flame or produce a weak signal, leading to nuisance lockouts. Choosing the OEM part ensures proper fit, a stable microamp reading, and rated safe operation across compatible Lennox, Armstrong, and Ducane furnaces.