Jump pin 1 to pin 2 and test for current flow. The brown wire at pin 5 seems to be useful for splitting the circuit, to test for voltage through the bi-metal.

Jumper Pins 1-2 on the ADC, check current draw of the defrost heater circuit. Perhaps disconnect Pin 4 depending on what it does, can't tell because of the picture being cut off.

It looks like the brown wire tells the board whether or not the bimetal is closed, by checking for the presence of 120VAC WRT Neutral, but I'm not completely sure. Also looks like there's a way to manually test that with a meter at the test terminal at the end of the wire.

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You're on the right track! What's the name for these kind of lines? 

Sensing lines!

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It doesn't matter what load pin 4 is supplying because you're only doing this long enough to test amps on the defrost circuit. But, this is another good extra credit question: even with a clipped off schematic, you should be able to discern the function of the switch with the common pole at pin 1. It's switching L1 from either the defrost circuit to something else. What would that be in a refrigerator?

Ah, makes sense. It's either cooling or defrosting, so that'd be to power the compressor and fans. 🧠

Jump pin 1 to pin 2. If heater comes on, ADC is bad. If heater does not come on, jump wires at test term. If heater comes on bad bi-metal, if heater does not come on, bad heater.

Brown pin 5 bypasses the bi-metal to check continuity on heater

Obviously it could always be a bad connection or a busted wire.