A gas furnace thermocouple is in place to shut off the gas line when the pilot goes out or the flow of gas is interrupted on the way to the pilot, it works as a type of sensor and if it fails your furnace will not light. A thermocouple is the right hand man of the pilot light as it reads the pilots temperature on a rating, which tells it to keep the gas flowing. If this temperature is not being hit due to a weak flame from the pilot, the thermocouple will shut off the gas flow to the pilot and shutting down your furnace.
Before checking the gas furnace thermocouple you want to make sure that you have the gas shut off by putting the pilot setting switch to off and waiting a few minutes. Once this is complete go ahead and remove the pilot cover which will reveal two upright posts, one is the gas line for the pilot and the other is your thermocouple.
During your inspection of the gas furnace thermocouple, you will be turning the gas line on and off while making a number of slight adjustments and tests to determine if the source of your pilot problem. Be sure to use your pilot lighting instructions on the side of the furnace while going through the following steps to insure an accurate inspection.
What you will do is simply do several simple on and off tests to check your gas furnace thermocouple for issues. First take a wrench and slightly make a tightening adjustment at the base of the thermocouple. Be careful as the area could still be hot from the pilot touching it and a tight pair of leather gloves can help protect your fingers from getting scorched.
Once you have adjusted the gas furnace thermocouple, go ahead and turn on the gas using the instructions located on the side of your furnace. If the pilot does not ignite, turn the gas switch off again, and wait the few minutes for gas dispersion.
To be sure the problem is coming from the gas furnace thermocouple and not the pilot, clean the pilot valve out with a fine wire and be sure that the outlet is not clogged from residue or build-up. Then turn on the pilot again following the lighting instructions on your furnace and watch the pilot for failure to light. If it does not light then you will then want to go ahead and unscrew the thermocouple with your wrench and remove it.
You can find a gas furnace thermocouple replacement at any good hardware store, or even on some on-line stores. Be sure you use the exact furnace details so you get the correct size replacement for your furnace. Most furnaces run on standard measurement fittings, while others have their own specialized specs and dimensions. Again this detailed information should be located on the side of your furnace, and if this is not available, you can gauge the male and female ends of your thermocouple at a good hardware store. This will help to insure you're getting the right size replacement for the thermocouple in your furnace.