How To Check the Thermocouple
The thermocouple is a safety feature in gas water heaters which determines if the pilot light is lit. The heat of the pilot light flame creates millivolt current in the thermocouple which energizes a magnet that in turn allows the gas control valve to operate. If the pilot light goes out, the thermocouple produces no power and the magnet closes the valve and will not allow the gas to flow. A common symptom of a faulty thermocouple is the pilot light will not stay lit.
Inspect the thermocouple by removing the inner and outer doors. Look for scale or corrosion on the small metal tube that leads from the gas valve into the burner area. The tip of thermocouple should be smooth and free of corrosion. The thermocouple should be suspended above the pilot light with the top half inch being positioned in the pilot light flame.
To check positioning in the flame when there is no flame, may require an assistant. First, wear safety glasses while attempting to ignite the pilot light. Press the pilot light button while observing the flame. When the flame is present, the tip of the thermocouple should not extend past the flame.
If the position is poor, turn off the gas supply and reposition the thermocouple to the proper place. Turn the gas back on and test the pilot light again.
Also check that the thermocouple connection to the gas valve is tight.
If the thermocouple appears to be properly positioned and in good condition it may still need to be replaced. Replacing the thermocouple should restore power to operate the gas valve and keep the flame going. If the problem still continues, the problem may be with the gas valve itself. If the gas valve fails, due to its high cost, it may make sense to replace the entire water heater, especially with older models.