The printer uses conductivity to detect the bed leveling washers. The washers are grounded to the bed frame and the bed frame has a wiring harness and the whole thing ultimately connects back to the control board.
The same is also true of the extruder nozzle. There is a grounding wire that goes from the hot end (I don’t own a Mini 2 but on my workhorse the wire is connected to one of the screws that attach the cooling fan on the side of the cold-end of the extruder). All this also goes back to the board.
When the extruder nozzle kisses the bed-leveling washer it completes the circuit and this is how the board detects that the nozzle has touched. Anytime this sort of thing happens, it means the circuit isn’t being completed.
MOST of the time, that’s caused by a dirty nozzle (a bit of plastic stuck to the end of the nozzle is acting as an insulator so the circuit is never completed).
Make sure the nozzle is clean. As @nopick says, use one of those scouring pads that were included in your printers toolkit – warming the nozzle to around 170-180°C will soften the material so it scrubs off more easily. Take care not to burn yourself … water boils at 100°C so you don’t want to touch a 170°C nozzle with your fingers.
I have a tiny brass brush (about the size of a toothbrush) that I occasionally use to the give the nozzle a good scrubbing – but take care not to let the wire bristles scrub at any of the wires near the hot-end (which is why it’s a good idea to have a tiny brush). I do this occasionally because I have a theory that a coating builds up on the hot-end over time (much like the “seasoning” that builds up on a cast-iron skillet) creating more resistance over time even though no visible plastic is on the hot-end. But scrubbing with the wire-brush will clean that off. (When I do this on my own printer it does seem to noticeably reduce bed-leveling probing failures).
If this doesn’t solve the problem, then you can check the conductivity using a multi-meter to measure the ohms.
E.g. if you measure the ohms from each bed leveling washer to the bed, they should have less than 1Ω resistance. When I did this on printer, I found 3 of 4 washers were good, but one had higher resistance (indicating a weak connection). In my case a washer was loose (they should each be snug but not so tight as to crack the glass).
You can also check the nozzle … put a probe on the tip of the nozzle and another probe on the aluminum heat-sink to check the resistance. This can indicate a dirty nozzle … but I’ve heard of metallurgical reactions between the nozzle and the hot-end that resulted in high resistance.
Again… it’s probably just a bit of plastic on the nozzle – it usually is. But if that doesn’t fix it, you can use a multi-meter to check the resistance of each component as you work your way back toward the board until you find the weak link … typically a poor connection somewhere.
