A thermocouple is a safety device found in a gas water heater that operates to automatically turn off the supply of gas to the water heater in the event that the pilot light extinguishes. There are two ways to determine whether the thermocouple is defective. The first is to examine the colour of the water heater's flame. The second is to inspect the tip of the thermocouple itself. Thankfully, making these determinations is a relatively simple task that can save you money by avoiding a professional inspection.
Remove the front access panel to the water heater.
Look at the burners to determine the colour of the flame. If the burner's flame consists of mostly a blue colour, the thermocouple is likely defective.
Turn the gas to the water heater off by twisting the gas control cock located on top of the heater's control box. The control box is located on the front of the water heater. Turn the control cock to the "Off" position.
Locate the thermocouple tube at the base of the control box. There are three tubes that run from the bottom of the control box: the pilot gas tube, the burner gas tube and the thermocouple tube. Each tube is of a different width. The thermocouple tube is the smallest of the three tubes.
Trace the thermocouple tube from the bottom of the control box to the inside of the water heater until the tip of the thermocouple is reached.
Remove the thermocouple from the interior of the heater. Note that the tip of the thermocouple located inside the heater leads to a large nut. Turn this nut in a counterclockwise direction with a wrench until the thermocouple is loose, then pull the thermocouple's tip away from the nut.
Inspect the tip of the thermocouple. If the thermocouple's tip is coated in ash deposits, it needs to be replaced.