Thermocouple replacement is a job that you can do it yourself and save. The average service call would cost you at least $100, but with a little help you can keep that money.
**NOTE** ONLY YOU CAN ASSESS YOUR ABILITY TO PERFORM THIS TASK. THIS IS A GUIDE AND CANNOT PROVIDE ALL THE DETAILS FOR EVERY SITUATION.
This guide will cover standing pilot furnaces and water heaters. Some furnaces/water heaters with intermittent pilots, will require extra steps to gain access to the burners and pilot. Most of them do not use a thermocouple.
To begin the replacement, you should shut off the gas and electricity to the furnace. Then you can remove the upper access panel of the furnace.The pilot assembly is usually mounted to one of the burners. To begin removal, you should disconnect the thermocouple from the main gas valve. You can find the connection by following the copper tube up to the gas valve. Next, you should disconnect the pilot gas tubing from the gas valve. This is the aluminum tube that runs from the gas valve to the pilot assembly.
Now, you can remove the screws holding the mounting bracket to the burner. This is usually two small screws. Then, you should be able to remove the whole assembly.
Once you have the assembly out, you should remove the thermocouple. Some of them screw out of the bottom of the bracket while others just snap in.
While you have the assembly out for thermocouple replacement, it is a good idea to clean the orifice. This is accomplished by unscrewing the gas tube from the bottom of the assembly. Then you should see a small cap-like piece called the orifice. This has very small holes in it that can become clogged. The best way to clean it is to just wipe the end of it off. **DO NOT TRY TO STICK ANYTHING IN THE HOLES**
After the assembly is cleaned, place the orifice back in and reattach the gas tube to the assembly. Ensure the connection is tight!
Thermocouples come in a variety of lengths but it is alright to replace one with a one of a longer length. Get a good universal type which comes with the adapters you could need for mounting it.
Place the thermocouple back in the mounting bracket and reinstall the bracket.
Next, you can reconnect the gas tube and thermocouple to the gas valve. Ensure that the gas connection is tight. Then you should turn the gas back on to the furnace.
Follow the instructions on the furnace/water heater for lighting the pilot. If there are no readable instructions, here is a general guide.
Turn the gas valve knob to pilot, then push it down. You may have to hold it down for a few seconds to get the air out of the pilot gas line. Use a long match and light the gas at the pilot outlet. Wait several seconds and release the valve knob.
Now, you should check the pilot gas connections for a leak and tighten as necessary. You can use a heavy soap solution or a commercially available gas leak detector. You should spray or brush the solution on the connections and watch for very tiny air bubbles to form.
Also, you should verify that the pilot flame is touching the end of the thermocouple. The flame should be blue and steady.
ENSURE ALL GAS CONNECTIONS ARE LEAK FREE!
Now, you can turn the gas valve knob to on. Turn the electricity back on to the furnace and run the unit through a normal heating cycle. Complete your thermocouple replacement by verifying proper system operation.