I just finished assembling my mk3. This is my first 3d printer. The self-test passes flawlessly, but the calibration fails as soon as the z axis gets near the bottom. I've tried re-tensioning / detensioning the belts and moving the pinda up / down. I am out of ideas on where to looks for the problem.
If it helps, here's a video of the failure:
https://www.youtube.com/watch?v=qZWOcVFbscg
Any suggestions on what part of the printer should be redone would be greatly appreciated.