Turn off the fireplace gas valve and remove the front panel from the fireplace. Some panels have a screw on each side that needs to be removed and other panels lower on a hinge.
Find the thermocouple, which is located next to the pilot light and looks like a small straight pipe.
Use a can of compressed air to gently blow away any dust and dirt from the thermocouple.
Wipe away any remaining dirt or soot on the thermocouple with a clean, dry cloth.
Close the fireplace access panel and restore the gas supply to the fireplace.