Precision Homing Failed  

Best Answer by hyiger:

Since it's a kit, I'm assuming this is the Core One and not the Core One L. 

Loosening or tightening the belts unevenly to square the gantry is what is causing this issue. Homing failed because the gantry is now skewed. To correct, try the following:

1. Loosen both tension screws until the pulley detaches from the screw, then reattach both tightening just until the screw is threaded 
2. Move the gantry to the front of the print and check that it is hitting the stops on both sides at the same time. If one side is skewed (there is play) then wedge something on the opposite side like the flat side of the universal wrench, then slightly bend the side with play by hand. Then recheck. 
3. Once square then tighten the tension screws. They must be tightened by the same amount alternating between them. So left 2 turns, right 2 turns. Do this until the belts start to tighten. Then left 1 turn, right 1 turn. Next left 1/2 turn, right 1/2 turn. 
4. Move the gantry and printhead to the center then move the gantry forward by hand checking that both sides still hit the stops at the same time. 
5. Next run the Manual Belt Calibration wizard. 
6. Once the belts are in the correct range with the upper belt 3-6 hz greater than the lower belt, homing should now pass
7. If the lower belt has a higher frequency than the upper belt, then go back to step one and redo the tension again. 

The key point is your gantry has to be square (parallel to the front of the printer) and both left and right tension screws must always be tightened or loosened by the same amount. Never turn them independently of each other.