OEM Lennox Flame Sensor — 33F37

The genuine OEM Lennox 33F37 flame sensor confirms that a flame is present after the burners ignite. Using flame rectification, it sends a small electrical signal to the control board proving combustion has been established. If the board doesn't detect this signal within seconds, it shuts off the gas valve to prevent a dangerous buildup of unburned fuel — making the sensor a key safety component of every heating cycle.

Common failure symptoms include a furnace that lights briefly then shuts off, repeated short cycling on ignition, the system going into lockout after several failed attempts, or no heat despite the igniter glowing. A dirty, corroded, or cracked flame sensor is one of the most frequent causes of intermittent furnace operation.

Why OEM matters: The 33F37 is built with the exact rod length, mounting, and material Lennox requires for a reliable flame-rectification signal. A genuine factory sensor ensures consistent detection and proper positioning in the flame. Aftermarket sensors may sit incorrectly or generate a weak signal, leading to nuisance lockouts. Install the OEM 33F37 for safe, dependable ignition and steady heat.