A gas fireplace is equipped with a thermocouple, which is a heat sensor that monitors the pilot light to prevent a gas leak. If the pilot light goes out, the thermocouple activates to shut off the gas valve as an essential safety feature. If your gas fireplace will not ignite or stay lighted, testing the thermocouple helps you narrow down the problem. A thermocouple is typically one of the first components to wear out in a gas fireplace due to its constant exposure to heat.
Shut off the gas to the fireplace by closing the valve in the line connected to the fireplace burner. Turn the lever on the gas valve 90 degrees clockwise to shut off the fuel. Wait for the fireplace to cool.
Remove the cover plate at the bottom of the fireplace with a screwdriver to locate the gas valve and pilot light, which is typically an L-shaped tube. The thermocouple is a thin copper tube between the pilot light and the gas valve.
Wipe the thermocouple with a clean rag to remove soot and grime that can interfere with the normal operation of the part.
Open the gas valve to release fuel to the fireplace.
Press in and turn the gas control knob for the fireplace to the ignite position. Press the ignite button to light the pilot while continuing to hold in the control knob.
Release the gas control knob after five minutes. If the pilot light continues burning, the thermocouple is working properly. If the pilot light goes out, the thermocouple is defective and must be replaced.