A gas fireplace thermocouple is a safety device that measures heat and recognizes if a pilot light has gone out, shutting down the fuel supply (such as gas or propane). If your pilot light stays lit for a few moments, but continues to go out after relighting, it may very well need the thermocouple replaced. Replacing the thermocouple is quick, and something you can easily do yourself.
Be Sure of the Problem
First make sure that the issue is with a faulty thermocouple as opposed to a dirty one. If there is debris in the gas pipe, this will also cause the pilot light to fail to remain lit, as it limits the amount of oxygen going to the pilot. Blowing out the pipe with some canned air usually does the trick and it will save you a trip to the hardware store.
Replace the Thermocouple
If blowing out the dirt does not help the pilot to remain lit, proceed to replacing the thermocouple. It is quick and easy to do, but perhaps the hardest part is just finding it among the logs in your fireplace. Make sure before you proceed further that you know exactly how the thermocouple is placed in the fixture so that you can place the new one the same way. Take a picture on your cell phone if you need a visual reference.
Turn the gas off to your fireplace entirely and make sure to give the thermocouple about half an hour to cool off before trying to handle it. Take a hex nut driver or screwdriver and loosen the objects holding the two thermocouple wires in place. Some models will use hex nuts while others will simply install screws.
Pull out the part with a pair of needle nose pliers and take it to the store with you so you know which replacement to buy.
Go in with a clean cloth wetted with isopropyl alcohol and clean the terminals you loosened earlier before trying to install the replacement. It also can’t hurt to be sure the ends of the wires on your new thermocouple are clean too. Then, attach the two new wires to the terminals and tighten the screws or nuts again with your driver. Place the new thermocouple in the same place your old one sat and close up the fireplace to conduct a test.
Turn the gas back on and relight the pilot light. Give the thermocouple about 15 minutes to heat up and then turn on the flame to make sure it all stays functioning.