American Water Heater BFG6140S403NOV

I am troubleshooting this water heater. The pilot light won't stay on and I suspect it is the thermocouple. I measured the voltage while running the pilot manually, highest it gets to is 18.1 millivolt. General consensus seems to be under 20 is bad.

I am wondering if I can confirm this is the only issue on the unit before tearing everything apart. In theory could I somehow apply voltage from a different source to the thermocouple or is that a dumb idea? Mainly to see if I can hear the solenoid opening, or if the pilot would stay running once I let go of the pilot button.

water heater label highest thermocouple reading multimeter leads

  • 2
    it's time to fix the shutoff valve as well
    – fraxinus
    Commented Feb 7 at 8:47
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    – FreeMan
    Commented Feb 7 at 12:56

1 Answer 1


In theory yes if you had an appropriate DC source lying around you might be able to use it to operate the gas valve. Note that the resistance of the solenoid may be very low and its current demands extremely small, and that the 20+ mV open-circuit thermocouple voltage will fall to a much lower value when loaded by the valve solenoid. An external voltage supply for test purposes may need to have a relatively high source impedance to mimic this effect and avoid over-current damage to the solenoid coil.

You won't hear the solenoid opening; the force to open it is provided by the human pushing and holding the pilot knob. The thermocouple provides power only to hold the solenoid after it is already pushed.

It appears the correct replacement part for your water heater is an assembly including the thermocouple, pilot burner tube, igniter, and possibly even the main burner. They're relatively expensive so I can see why you might want to test the valve first.

You could pick up a much cheaper universal thermocouple, test the gas valve with that, and then make a decision about buying the correct replacement part to repair the water heater properly.

  • 3
    Thank you, reading this gave me an idea I did not realize. I have already purchased a universal thermocouple, but I was thinking I would have to open the burner assembly to test it. In theory I don't! I can just install it on the gas valve, keep it outside of the burner, and then take a lighter and run it on the end and see what it does I believe. It should generate the required voltage and operate as if it was sitting in the pilot light.
    – John Smith
    Commented Feb 7 at 16:40

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