Great question — answering as the UphillTech team:
You've got a good start on troubleshooting already. Here are a few more things to check. With the thermopile disconnected, measure the resistance across the gas valve terminals. It should be no more than a few tenths of an ohm. (Since many ohmmeters show 0.2Ω to 0.5Ω with test leads shorted, there should be no more than an increase of a few tenths of an ohm above that shorted measurement.) If the resistance is much higher, or open, the valve is at fault and must be replaced. Are the thermopile leads making good contact with the valve terminals? To be sure, measure the voltage across the valve terminals, not across the thermopile leads, when checking the hot output. Since the measurement is...
Full answer by DrMoishe Pippik: https://diy.stackexchange.com/a/332271 (CC BY-SA 4.0).
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