You will need a multimeter and access to your water heater to test the thermocouple. Make sure the pilot light has been off and cooled down before you open the doors to access the heat element assembly.
Why should you test a thermocouple?
As a reminder, the thermocouple is an electrical device that looks like a wire and screws into the gas valve of the water heater on one end while being heated by the pilot flame on the other end. By using two different sized conductors, the thermocouple can translate the heat into a current which disappears when cold, thereby closing the gas valve. Without a working thermocouple, you could have gas flow while the pilot light is out, risking dangerous amounts of toxic and flammable gas to build up.
Checking the thermocouple
If you are checking the thermocouple because you suspect that the pilot light has gone out, note when you first see the assembly whether the thermocouple has just wiggled out of its proper connections.
If the pilot light will not stay lit, the conductivity of the thermocouple should be tested immediately using the multimeter. Attach one of the probes to the copper sheet of the thermocouple and the other to the gas valve. You will also need to know what type of gas valve you have, either White-Rodgers or Robertshaw.
For a White-Rodgers valve the reading should be 10 millivolts.
For a Robertshaw valve, the reading should be 13 millivolts.
If your reading is lower than that approximate value, you should replace the element. You could either use the specific design that fits your water heater or rely on a common universal thermocouple.
Replacing a thermocouple
Unless you have extensive plumbing experience, we don’t recommend trying to install a new thermocouple yourself. This small, delicate part deals with electricity and natural gas, so any small mistake can be very dangerous. Also, if the thermocouple is not installed correctly, it could cause damage to your water heater.