A limit switch protects the Lennox G40UH(X) gas furnace from overheating. If the G40UH(X)'s limit switch detects a temperature above its set range, the limit switch opens. When the limit switch opens, the G40UH(X)'s control board turns off the furnace's gas valve and turns on the furnace's blower motor. Once the furnace temperature returns to a normal operating range, the limit switch automatically closes. A replacement limit switch must match the original limit switch's temperature rating: the G40UH(1) through G40UH(6) units use a 125 F limit switch and the G40UH(7) uses a 135 F limit switch.
Step 1) Turn off the Lennox G40UH(X)'s circuit breaker.
Step 2) Turn off the building's gas valve, using an adjustable wrench to turn the valve's knob counterclockwise until the knob touches its backstop. The gas valve's intake port connects to the end of the building's gas pipe and is located outside of the furnace. Smell the gas valve. If the gas valve has a gas odor, call a licensed contractor immediately.
Step 3) Lift up on the Lennox furnace's upper access panel. Pull the bottom of the upper access panel away from the furnace. Set the upper access panel aside. Remove the lower access panel, using the same method used to remove the upper access panel.
Step 4) Locate the faulty limit switch. The primary limit switch, located above the burners and to the left of the furnace gas valve, mounts against the G40UH(X)'s housing. The secondary limit switch connects to a bracket near the G40UH(X)'s blower motor.
Step 5) Pull the wire connector off the faulty limit switch. The wire connector slides over the limit switch's terminal prongs
Step 6) Remove the mounting screws holding the limit switch in the G40UH(X), using a 5/16-inch hex screwdriver to turn the screws counterclockwise. Slide the faulty limit switch out of its slot in the G40UH(X).
Step 7) Insert the new limit switch into the unit's limit switch slot. Tighten the limit switch's mounting screws, using the hex screwdriver to turn the screws clockwise.
Step 8) Align the furnace's limit switch wire connector with the limit switch's terminal prongs. Push the connector against the limit switch until the connector snaps into place.
Step 9) Hold the top of the bottom access panel against the Lennox G40UH, keeping the bottom of the panel pulled away from the furnace. Push the bottom of the bottom access panel against the furnace. Slide the bottom access panel down into its slot in the furnace's housing. Install the top access panel, using the same procedure used to install the bottom access panel.
Step 10) Turn on the building's gas valve, using the adjustable wrench to turn the valve's handle clockwise until the handle touches the backstop. Smell the gas valve. If the gas valve has a gas odor, call a licensed contractor immediately.
Step 11) Turn on the Lennox G40UH(X)'s circuit breaker.
Some local and state building codes may require that a licensed contractor perform this furnace repair. A technician will troubleshoot the furnace limit switch and give the Lennox G40UH furnace a complete diagnosis before replacing the limit switch. In many cases a faulty limit switch is a secondary issue that develops after the furnace's primary fault has developed.
Lennox: Service Literature Unit Information G40UH(X)