A thermocouple is a safety device in a gas water heater which operates to automatically shut off the gas when the pilot light extinguishes. Over time, the tip of the thermocouple will develop a build-up of ash, which prevents the thermocouple from properly functioning. A pilot light which is mostly blue or which will not stay lit are common symptoms of a defective thermocouple. Thankfully, a replacement thermocouple is relatively inexpensive and can be installed in only a few minutes.
Shut the gas to the water heater off by turning the plastic gas knob on top of the control box to the "Off" position. Allow 15 minutes for any gas to dissipate.
Remove the burner chamber's outer and inner access panels. The access panels are located just below the control box. Once the outer access panel is removed, the inner access panel, which sits just behind the outer panel, can be removed.
Remove the thermocouple from the control box. The thermocouple is the smaller of the two metal tubes attached to the bottom of the control box. The thermocouple can be removed from the bottom of the control box by first turning the nut at the end of the tube in a counterclockwise direction with a wrench, then by pulling the thermocouple away from the control box.
Remove the thermocouple from inside the water heater. Trace the thermocouple tube into the water heater until the end of the tube has been reached. Note that the end of the tube leads to a nut. Turn this nut with a wrench in a counterclockwise direction until the tube is free from the nut, then pull the thermocouple out from the water heater.
Install the new thermocouple into the inside of the water heater. Place the tip of the thermocouple into the nut located inside the water heater, then turn the nut in a clockwise direction.
Install the new thermocouple into the bottom of the control box by placing the threaded nut over the opening at the bottom of the control box and turning the nut in a clockwise direction.
Reinstall the inner and outer access panels and light the pilot to complete the replacement.