Lennox gas furnaces contain an important safety component known as a flame sensor. As long as the component senses the flame, it sends a message to the main furnace control board telling it to keep the main burner on and the gas valve open. If the flame sensor can't sense the flame because it's too dirty, the furnace may shut itself off. Finding and cleaning the flame sensor in a Lennox furnace is a simple repair that takes just minutes.
Flip the circuit breaker switch that corresponds to your Lennox furnace to the "Off" position.
Lift the access panel on the front of the furnace straight up to clear the tabs on the bottom of the furnace cabinet. Set the remove panel aside. Note that some Lennox furnace models require you to remove two side screws to free the front access panel.
Look for a thin metal rod on the left side of the burner assembly; this is the flame sensor. Remove the screw that holds the flame sensor rod in place.
Scrub the flame sensor with steel wool to remove any grimy build-up. Wipe the sensor with a clean tissue before replacing it to remove any remaining residue.
Replace the front access panel. Flip the corresponding circuit breaker switch back to the "On" position to restore power to the furnace.