1) Still thinking endstops - if you push the print head all the way to the corner do the endstops click? Maybe they are loose and need to be moved back towards the center a tiny bit?
2) The other common problem is the pulleys that the belts go around. Sometimes the setscrew in the pulley gets loose. Your UM2 should have come with a screwdriver that fits these. Make sure they are all tight. Very tight. There are 6 per axis, 4 on the long belts, and 2 on the short belt to the motor. If you don't find a loose pulley you should remove the cover over the motor to get at that pulley also. I usually recommend using a permanent marker to make a little dot on the pulley and the shaft so that if it moves you can see it happening. You could instead try holding the short belts still and try to force the head to move to see if a pulley is slipping but this doesn't test the pulley on the motor.
I can confirm one of the endstops is broken. I found this out playing around with OctoPrint where I have a terminal receiving the messages of the printer. When moving the head to the left, it always says "endpoint reached". After that I used a multimeter to check the endstop pressed and unpressed - and I got almost the same values (in contrast to the other endstop with clear 1 - 0 values).
Thanks for your help!
It's probably just a loose wire. The shipping on the UM2's seems to have been unusually rough and some connectors came off. Take the larger cover off on the bottom of the UM and check all the connectors.
I think it's the connector with 2 blue wires in the top in this photo where it says "x-stop".
Sounds like an issue with the mechanical end stops, in the backleft top of the machine there are 2 switches with red and blue wires. If you press those, do they still click?
This could happen if they are permenently "fixed" to closed.
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