Problems with the Y-Calibration  

RE: Problems with the Y-Calibration

I’ve got the exact same problem too! It also is happening when unloading filament and it attempts to ‘park’ the extruder. My new printer is just junk at this point. 

RE: Problems with the Y-Calibration

Check to make sure your gantry is still parallel to the frame. Release the belt tension so it is slack. Move the gantry all the way to the front and check that it hits the end blocks at the same time on both sides. If not, then use a wrench to pry the opposite side and then slightly manually bend the side with a gap it until it is square. When retightening the belts, ensure it's done slowly and evenly or can end up bending the gantry out of square again.

If the gantry is already squared, then check under the left hand side rail that the bearing is not colliding with a loose cable

RE: Problems with the Y-Calibration

also consider getting in touch with Prusa's support. It may take long but they will help

RE:

Call it a hunch, but the Y rod alignment seems iffy, and depends on too many screw and punched hole variables. With the printer powered down, use a scale and pull the gantry from the back to the front and see if any spots are sticky or require more force. If the rods are not parallel there will be excessive drag towards the front or rear of the frame. You can also feel for stiction: place the gantry towards the read, and feel for how much force it required to start it moving front to back. Then move the gantry to the front, and feel for that same amount of force to start it moving. It should be identical, regardless of where on the Y-axis you are starting from.

And, did you properly pack the bearings with the provided grease before assembly? If not, tearing the printer down to do so is in your future.

RE: Problems with the Y-Calibration

As above verify that the two sides of the gantry are properly seated into the smooth rods.

However for me it kept failing because of the way I assembled the filament sensor entry point on the right - I had followed the instructions exactly but the Y carriage was tipping off the cabling connecting the filament sensor. So I broke with the instructions and placed the cable on the right of the pfte tube rather than the left, tightened it neatly, and this solved the problem. I'm not clear why the instructions here show the filament sensor cable floating on the left of the pfte tube as its difficult to get it to neatly pull down into the zip tie from that position where it can get in the way of the Y carriage.

RE:

I just posted about this, and fixed it: https://forum.prusa3d.com/forum/prusa-core-one-assembly-and-first-prints-troubleshooting/y-axis-calibration-failure-new-kit/

I'm going to work on a thorough post about this issue, because I don't think it's really explained in the instructions on how this all actually works and what needs to be done with the belts.  But in the meantime ...

Here you see the gantry knocking into the tension pulley instead of the motor mount.  The problem is that you need to retract the tension pulley before you start the alignment process.  However, if you inserted too much of the belts into the Nextruder holder (e.g. more than 4-5 teeth, as described in step 5.57), then when you retract the pulleys, the belts will be too tight and you will fail the belt tension test.  

Unlike the previous generation printers, the X- and Y- belts are interconnected, and the Y-axis test is affected by the belt tension (because of the pulleys).  

In your video, did you notice that you failed the Y-axis test just as the gantry hit the back of the printer?  That's because the gantry could not move far enough along the Y-axis as it should.  And that was probably because the pulley blocked the gantry from starting where it should -- it was 2mm too far forward.