Coleman makes a variety of gas furnaces designed for high-efficiency heating of residential homes. All their models of gas furnaces consist of pilot light assemblies that include a thermocouple. One end of the thermocouple on a Coleman gas furnace is connected to the incoming gas supply line. The other end is connected to the pilot light. The thermocouple shuts off the gas to the pilot if it senses no heat from the pilot. If there is no gas supply to the pilot light, the thermocouple may have failed and should be replaced.
Trace the gas line from the furnace until you reach a shut-off valve. Close the shut-off valve by turning the handle or knob completely clockwise.
Remove the pilot light access panel, which is located near the bottom half of the furnace. Lift up on the panel and pull it off the furnace.
Locate the thermocouple connected to the pilot light. Remove the nut that secures the thermocouple to the bracket on the pilot light using an adjustable wrench. Pull off the thermocouple from the bracket.
Unscrew the nut that secures the other end of the thermocouple to the incoming gas supply line using an adjustable wrench. Completely remove the thermocouple and dispose of it.
Attach the larger fitting on the new thermocouple to the incoming gas line where you removed the old one. Connect the other end of the new thermocouple to the bracket by the pilot light. Ensure both connections are tight. Replace the access panel, open the gas shut-off valve and fire the furnace.