The thermocouple on a gas appliance, such as a Heat & Glo fireplace, is a safety device that shuts off the gas when the pilot goes out to prevent the accumulation of unburned gas around the fireplace. A thermocouple works by sending a tiny electrical current to the gas valve whenever the probe, which is next to the pilot, is being heated by the flame. When a Heat & Glo thermocouple wears out, it's a not-too-difficult job to replace it, the complicating issue being lack of access to the underside of the grate where all the gas paraphernalia is located.
How to Know If the Thermocouple Needs Replacing
It's possible to test a thermocouple with a multimeter to determine whether it's sending a current when the pilot flame is on, but you might not need to go to that much trouble. If the thermocouple is bad, the pilot won't stay lit, and there's an easy way to distinguish a thermocouple malfunction from another fault that prevents the pilot from lighting, such as a clogged pilot tube.
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If the thermocouple is bad, you'll be able to light the pilot by following the normal ignition sequence. It will stay lit as long as you hold in the gas control, but as soon as you release the control, the pilot will go out. If clogged gas tubes are causing your pilot problems, the flame won't start or will be weak and burn unsteadily with an orange hue, and it will keep burning when you release the gas control.
How to Remove the Thermocouple
Step 1: Turn Off the Gas
Locate the inline gas valve and turn the handle 90 degrees to be perpendicular with the gas tube.
Step 2: Disconnect the Thermocouple From the Gas Valve
Look at the gas control module and you'll see three copper tubes attached to it. The two large ones on either side feed the burners and pilot, and the small one in the middle is the thermocouple. Unscrew the nut holding it to the module using a small wrench and pull it out.
Step 3: Disconnect the Probe From the Pilot Assembly
Follow the thermocouple tube to the other end where the probe is mounted next to the pilot orifice. You'll see a small screw on the mounting plate. Turn this screw with pliers and when the thermocouple is loose, you can lift it right out of the fireplace.
Testing a Weak Thermocouple
If a Heat & Glo thermocouple is weak but hasn't failed completely, the fireplace may work only intermittently. To determine if the thermocouple is really the problem, set a multimeter to measure voltage in millivolts (the mV setting on the meter dial) and make sure the pilot is burning. Place one of the meter leads just under the thermocouple probe, which should be in the pilot flame, and the other lead on the nut that connects it to the gas valve.
If the thermocouple is good, you'll get a reading between 25 and 30 mV, and if it has failed completely, you'll get a reading under 20 mV. A reading somewhere between 20 and 25 mV indicates that it's weak, and the closer the reading is to 20, the more urgent the need to replace it.