Thank you, SanderG, for your kind recommendation, I respect your opinion.   Anyway I just repaired it, my UM3 ist printing again. Wasn't so difficult, please see my photos and the description.   Both of the wires connecting the capacitive leveling sensor are plugged into contact springs on the sensor, and the other side is plugged into a print head PCB connector. Obviously there is a design problem which leads over time to breaking wires at the sensor side, due to bending the wires when opening the print core cover for changing the print cores.   If you want to repair the broken leveling sensor wires, you got to unscrew the four screws holding the bottom part of the print head.   The bottom part then is only held by the level sensor cable and the front fan cable (the two middle connectors of the four ones).   Disconnect both of them, be careful to not damage the wires while freeing them.   Now that you have the printhead bottom part separated from the printhead, you can disconnect the wires (or remove the broken part of the wires) from the contact springs by carefully opening the inner part of the spring until the wire can be pulled out. Don't use too much force, the contact spring is very delicate! If you bent it away from the middle you gotta bend it back. You might wish to use a magnifier and a pointed tool like a needle for this part of the action.   Un-isolate the ends of the wires and solder them, then plug them back into the contact springs, take care not to reverse them (see pictures). Check that they are tightly stuck in there, otherwise you need to bend the spring somewhat more.   After re-assembly of the printhead (careful with the side fans and their cables) and inserting one or both of the print cores you can start the auto-leveling procedure and hopefully start printing again.   Good luck!    
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