Troubleshoot - Bunn STF Runs On It's Own


New member
Nov 24, 2020
I've an older Bunn STF (with a dial timer board) plumbed coffee maker that the maker trips on it's own, allowing water to flow in continuously.

When first turned on in the morning, it makes coffee correctly, no problem, but after a while, say 30-60 minutes, the system trips to make again.
The water level sensor is new, and it does kick off when water reaches it normally (after draining some water from the spout).

Followed the manual's testing and found the following.
On the timer board check, it says the voltage across TL1-TL4 should be 0, and I read 0.4VAC. Not quite 0, but don't know if this is an issue.
It does change to 120V when start button is tripped, like it's supposed to.

On the liquid level board check, it says across terminals 1-3 it should switch to 120 after a 5 second delay, which it does, and it should go to 0 when terminal 4 is shorted (or water level is reached), but I read ~3VAC.

All other tests match the diagnostics.

When it trips on it's own, the relay does trip, so one of the boards does seem to think a call to make or low water level has been triggered, and it's not just the solenoid leaking through.

Any ideas which section is at fault?
I could just shotgun both boards, but that's ~$150, and I'd rather just repair or replace the one board if possible.
Almost seems like it would be the liquid level board since it reads 3V rather than 0, but don't know enough about the circuit.

Thanks much.