How to Replace a Limit Switch on a Lennox Merit Series G40UH(X)


Lennox Merit series G40UH(X) appliances are mid-efficiency, natural gas furnaces available in upflow and horizontal models. These furnaces contain internal safety devices, known as limit switches, used to prevent the appliance from overheating. When a limit switch senses temperatures above the factory-set normal operating range, it sends a message telling the control board to shut down the hot surface ignition system and close the internal gas valve. Functioning limit switches reset within an hour; replace the limit switches in your Lennox furnace if they do not reset. Lennox Merit series G40UH(X) furnaces contain both primary and secondary limit switches.

Things You'll Need

  • Needle-nose pliers
  • Hex-head screwdriver
  • Flip the circuit breaker that supplies the electricity to the Lennox Merit series furnace into the "Off" position to avoid serious injury or death during the limit switch replacement process. Find the gas supply valve behind the furnace; rotate the valve counterclockwise as far as it will go to disrupt the gas supply.

  • Remove the two service access panels on the front of the furnace. Lift the top service access panel straight up while pulling the bottom of the panel toward you to disengage the retaining clips on the furnace cabinet. Repeat the process to remove the bottom service access panel; the side-by-side service access panels on horizontal models are removed in the same way. Lean the removed access panels against a wall; take care to avoid scratching the enamel finish.

  • Locate the primary limit switch in the furnace burner compartment; look just above the burner assembly and to the left of the internal gas valve. Find the secondary limit switch in the blower compartment, mounted to the left rear side of the blower housing.

  • Disconnect the wiring from the failed limit switch. Pull the slip-on wiring connectors with a pair of needle-nose pliers to free them from the limit switch terminals.

  • Remove the two screws mounting the limit switch component to the furnace housing, using a hex-head screwdriver. Discard the removed limit switch, but reserve the screws for reassembly.

  • Hold the replacement limit switch against the furnace housing and align the screw holes. Replace and tighten the reserved limit switch mounting screws. Reattach the disconnected limit switch wiring; guide the wiring connectors onto the limit switch terminals, using the needle-nose pliers.

  • Reverse the panel removal process to replace the service access panels. Turn on the electricity to the furnace from your circuit breaker box and open the gas supply valve.

Tips & Warnings

  • Replacing a failed limit switch in a Lennox Merit series furnace may void its warranty. Consult the terms of your warranty agreement for additional information.

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