The thermocouple on your gas fireplace is a heat-sensitive apparatus that keeps the pilot gas tube open while the pilot is lit. It does this by creating a small electric current — on the order of 10 to 30 millivolts — as a sign to the gas valve. Thermocouples wear and so are replaceable, but you should conduct a few straightforward tests before doing this, because your gas pilot could possibly be moving out for another reason. The thermopile is a connected apparatus you should test at the exact same time — its role is to open the gas to the burners.
Assess the Pilot
Locate the pilot, frequently behind a removable grill running across the bottom of the fireplace. Light the pilot with the recommended procedure for your fireplace. The typical procedure is to turn the gas valve to “Pilot,” push in the knob and then depress the red ignitor button until the pilot lights. If you have an older fireplace, you may need to light the pilot with a long match.
Hold the knob in for 60 seconds, then release it. If the thermocouple is functioning, the pilot must stay lit. If the pilot goes out, then relight it and check the flame. It should be about an inch in length and mainly bluish in color, and it should be in contact with the tip of the thermocouple.
Clean the pilot tube with compressed air, following the manufacturer’s instructions, if the pilot flame is irregular or small. Move the thermocouple tip nearer to the flame if it was not within the flame’s hit with the pilot was lit. Relight the pilot and also attempt the test again.
Examine the thermocouple with a multimeter when the pilot goes out again. Set the meter to measure millivolts and, to make measurement easier, fit the leads with alligator clips.
Locate the gas valve — it’s usually behind a grill on the front of the fireplace and discover the copper thermocouple cable leading from it. The best way to do this is to follow the cable back in the tip of the thermocouple, which is right next to the pilot light. Unscrew the thermocouple fitting and eliminate it in the valve, using a 7/16-inch wrench. Catch the pilot end of the thermocouple in place.
Attach one lead of the multimeter to the silvered finish of the thermocouple cable (the component that made contact with the gas valve) and the other lead to the bare copper thermocouple cable. Light the pilot, then be sure that the flame is connected with the thermocouple tip and then hold from the gas valve for 60 seconds. You should get a meter reading of 20 to 30 millivolts (or within the very low range of the thermocouple’s score; voltages vary by product). If the reading is less than 20 millivolts, it’s time to replace the thermocouple.