The gas system in a propane or natural gas log fireplace works much in the same way as the burner for a water heater. Both systems make use of a heat-sensitive thermocouple, whose function is to detect the pilot light and keep the gas valve open so the pilot can stay lit. If you can't get the pilot on your fireplace to stay on, the thermocouple could be bad, but the problem could also be a blocked pilot tube or a malfunctioning gas valve, although gas valves rarely malfunction. You can easily test both the pilot and the thermocouple.
Check the Pilot Flame
Before checking the thermocouple -- which isn't an onerous procedure, but may be unnecessary -- look at the pilot flame. To see it, you'll first have to locate the gas valve -- it's usually behind a panel on the front of the fireplace, near the floor. Turn the control to "Pilot" and light the flame by pressing the piezoelectric ignition button; you should be able to see the pilot behind the valve, but it may be buried inside the logs somewhere -- you may have to look for it. It should be burning steadily with a bluish light -- flickering or a yellow tinge are signs of a weak flame, usually caused by debris in the pilot tube. A weak flame won't drive the thermocouple, and the pilot probably goes out shortly after you light it.
Clean the Pilot Tube
If the pilot light is weak, the pilot tube by unscrewing it from the gas valve, using a 7/16-inch wrench, and blowing it out with compressed air. You may also have to remove debris from the orifice with a pin.
Check the Thermocouple Operation
If the pilot flame is burning with a bluish color, and it's about an inch high, the thermocouple should send an electric signal to the gas valve to keep it open. It may not be doing this if the pilot goes out as soon as you release the gas valve. To confirm the need to test the thermocouple:
- Set the gas valve to "Pilot," light the flame and continue depressing the knob for a minute before releasing it.
- Move the tip of the thermocouple closer to the flame. You can do this by sliding the copper tube to which the tip is attached forward.
If neither of these procedures results in the pilot staying lit, you can perform a simple test to determine whether or not to replace the thermocouple.
Testing the Thermocouple
You'll need a voltage meter capable of accurately measuring voltages in the 1 to 200-millivolt range. A multimeter will do. The leads should have alligator clips -- if not, you may need a helper to conduct the test.
Locate the thermocouple tube -- it's made of copper and is usually attached to the front or side of gas valve. It sometimes looks like the pilot tube -- distinguish it by following it with your eyes to the sensor tip.
Unscrew the thermocouple from the valve, using a 7/16-inch wrench. Pull it away until you can see the silver soldered tip that fits inside the orifice in the valve.
Attach one of the meter leads to the soldered tip and the other lead anywhere on the copper tube. If you don't have alligator clips, you'll have to hold the leads while your helper lights the pilot.
Light the pilot and hold in the knob to keep it lit. Wait for about 30 seconds, then check the meter. If the pilot is good, the meter will read 25 millivolts or more. If the reading is less than 20 millivolts, replace the thermocouple. The thermocouple may still be functionally adequately if it measures in the 20 to 25 millivolt range.