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This unit doesn't have a thermocouple but you're on the right track, that it's not detecting the flame. Instead of a thermocouple this unit has a flame sensor (see attached pic for similar). You can try cleaning off the metal rod of the flame sensor with fine aluminum oxide sandpaper or steel wool. Also vacuum out the combustion chamber since a stray rust flake can also foul the flame sensing. Also check the electrical connections on the flame sensor lead. If that doesn't work the next thing to try is to replace the flame sensor.