Kill pin (“fault detected”) getting triggered during calibration and homing

When it rains, it pours…

Now i’m getting a completely new and bizarre issue.
I can’t finish a calibration properly anymore, because the kill pin keeps getting triggered, as if the effector was at an angle (it’s not).

It usually happens near the bottom of the range of motion.
About 1 in 5 attempts, i’ll actually get through the calibration.

What on earth is going on?!
EMI?

It gets weirder…

When i do manage to calibrate, bed leveling seems completely off - consistently sloping up to X and Y, more towards Y tower. I’ve replaced the piezos - no change. I even tried the glass side, and tried a fresh BuildTak - again, no change.

It’s like the whole printer decided to spontaneously and gradually self destruct over the past week.

Took the hotend PCB off, the underside was pretty doused with flux, and the flux around the accelerometer QFN package seemed to have absorbed the moisture. I’ve cleaned that out with a q-tip and ethanol (ran out of IPA), and it looks like the faults have stopped for now.

Still too early to tell if that was the cause, but it’d actually make sense, as it’s been very humid lately.

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