How to Replace a Lennox G12 Thermocoupler


The Lennox G12 furnace comes with internal components standard to most home furnaces. One of these components is the thermocouple. This device works in conjunction with the pilot light and senses if there is enough of a flame to start the unit. If the thermocouple doesn't detect a pilot flame, it shuts off the gas valve. This is an important safety feature, so if the thermocouple in your G12 is broken you need to replace it immediately.

  • Locate the power switch connected to your Lennox furnace and switch it to the "Off" position. You can also turn off the circuit breaker that controls the furnace. This will be located in your home breaker panel.

  • Slide the front panel off of the furnace. You now have access to the thermocouple.

  • Unplug the copper lead from the thermocouple assembly.

  • Unscrew the gas connection line from the thermocouple assembly.

  • Locate the thermocouple tube nut below the mounting bracket. Unscrew it.

  • Remove the thermocouple assembly from the furnace.

  • Fit a new thermocouple assembly in the furnace with the steel tube facing up and the copper tube facing down.

  • Tighten the mounting bracket nut and connect the copper and gas lines.

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