If you have a multi-meter, check the ohms between the nozzle (make sure the nozzle is clean) and heat-sink. The “cold” end of the extruder has four screws around the aluminum heat sink. one of the screws has a red wire attached. That is the “zero sense” wire used to conduct back to the board to detect when the nozzle touched a bed washer.
If you measure the ohms between the nozzle and the red wire, you should read less than 1Ω (my printer typically reads around 0.7 to 0.8). If it’s reading high … then something is acting as an insulator.
What can do this?
The extruder has a “hot” end (the heater block), a “cold” end (the heat sink where you see the fan attached), and a “heat break” that separates the hot and cold ends with a threaded tube running between the two halves.
The “heat break” threads into hot-end from above… the nozzle threads on to hot-end from below. But the two threaded parts should come together in the middle of the heat break and they create a seal.
IF they aren’t sealed, filament can leak through the gap … make it’s way through the threads (remember it’s in a melted state and under a bit of pressure) and this reduces the conductivity because they aren’t solidly touching … with melted filament acting as an insulator.
You would be able to tell by inspecting the “heat brake” (the gap between hot-end and cold-end of the extruder). If you look in that gap … you should just see the threaded tube … no plastic. If you see melted filament in there… that means it has been leaking and it needs to be cleaned.
This did eventually happen to me and I had to clean mine. The process is moderately tricky… you can’t just grab a wrench and take it apart.
It’s difficult to inspect the heat-brake with the part-cooling fan duct attached. The bottom two screws on the aluminum heat-sink are also holding the fan-duct in place. Remove those two screws … BUT … the fan will come with the fan-duct and the fan is plugged in. You don’t have to disconnect the wires … you can let it dangle, just be gentle so as to not rip out any wires. Also … the screws are not the same length so remember which screw was in which corner.
With the fan duct removed, you can inspect the heat-brake and make sure it you don’t see any melted filament in that gap. If you do… this is likely why you are having un-reliable bed leveling.
I’ll leave the cleaning instructions for a different post here because if your printer is still under warranty, you might want to call support rather than attempt to clean it on your own. I did clean mine on my own (and it’s under warranty) but I was comfortable with the process and had all the tools needed.
The process is a little tricky because the extruder nozzle and heat-brake can only be disassembled with the extruder “hot” at 280°C … attempting to unscrew it while cold often results in the threaded heat-break tube shearing and then you have to replace it. There’s a bunch of steps required and some steps require heating up the extruder. This was enough work that I think the average owner should think twice before deciding to try this on their own. This is why I suggest that if your printer is under warranty… call Support.
If you are no longer under warranty and you want to undertake this yourself, let me know and I’ll provide the steps I used to clean mine. It is a bit tricky to do and you also need a decent soldering iron (with the heated tip scrubbed clean).