Today my bed levelling started to fail, and I am 99% positive its not a caked nozzle.
I tested the voltage with a DMM and I’m not getting anything between any part of the (squeaky-clean, gleaming) nozzle and any of the four bed washers, but have good voltage when I test the connection between any two bed washers.
So I think something has gone awry with the wiring on the nozzle side.
Which of the four wires connecting to the nozzle/hot end are responsible for the bed-levelling voltage?
Happy to re-solder or bridge, just need to know where to start. The OHAI doesn’t appear to have clear info on that, unless I’m missing something.
Many thanks for any pointers! 
Those four wires sound like the two pairs of wires for the heater cartridge and thermistor. There should be a single wire with a terminal ring screwed into the side of the heater block (on a Mini 1) or the face of the heatsink (on a Mini 2).
I’m not sure you understand what you are measuring with the meter. You should measure nearly zero Ohms resistance between all the washers and a ground connection. You should measure nearly 5 Volts between the nozzle and the washers with the machine powered up. Don’t measure resistance here.
The most common failure point for the leveling sensor connection is the “ground” wire attached to the underside of the metal bed plate. The crimp ring connector can loosen from the wire or the wire can break from wear at the crimp. I’ve had two of them fail on two Minis. I soldered the wire into the ring connector on both and have had no failures since. Here’s what the original connection looks like:
This seems to be the issue. The connection is actually fine on inspection, so I think the cable is broken. I’m going to need to bridge it, but am unsure where it terminates on the RAMBO.
https://ohai.lulzbot.com/media/uploads/CBfanplugin.jpg tells me nothing… Any idea?
Never mind. “Z min”. Of course. Oh boy, that was one long brainfart on my part…