Close the valve in the gas line connected to the Lennox. Turn the handle on the valve one-quarter turn clockwise to stop gas flow.
Turn off the circuit breaker to the room wired with the Lennox fireplace and wall switch.
Remove the screws holding the switch faceplate to the wall. Pull off the faceplate and remove the screws holding the switch in the mounting bracket underneath.
Pull the switch forward to loosen the screws in the sides and take out the two wires connected to the screws.
Set the ohmmeter to the low resistance mark, typically labeled "Rx1" on the device. Hold the two test probes on the ohmmeter against the two screw terminals on the switch.
Dial the switch to the "On" position. If the ohmmeter continues to register zero, the switch is defective and must be replaced.