M300 Plus Endstop X Homing Failure

Support&troubleshooting
1

We’ve encountered a weird homing failure after the last major maintenance at just shy of 3000 operating hours.

Trying to finish maintenance by auto-calibrating the platform, the printer starts homing. The Y-Axis works as expected, then for the X-Axis, the extruder stops at the correct position (as expected), but approximately a second later, it continues to travel to the left, hitting the frame and causing Error #5.5: Endstop X Homing Failure.

The axes are parallel, belts are properly tensioned and the extruder can be moved by hand without resistance. I’ve also placed washers beneath the optical endstop sensor, to eliminate optical problems of the sensor, but the result is the same: It stops in front of the sensor, then continues its travel after a second crashing into the frame.

There have been no Firmware updates since the last successful print before the maintenance (FW v. 2.6.15).

Has anyone encountered something like this before, or has any idea on how to troubleshoot this?

EDIT: A video (gif because of forum restrictions) showing the process:
M300Plus_XHomingFailure-2025-09-30-185213

2

So we’ve managed to finally solve the problem.

It turns out, that the short x-axis motor belt was a bit too loose, despite the motor being pushed down during mounting, and therefore the homing process couldn’t move back the extruder enough before continuing forwards again. Probably because of the position in which the axis was mounted plus maybe lengthening of the belt, it was just outside the tolerances.

So I dismounted the motors, loosened the screws of the axis bearings, front plates and guide rails and fastened them again, but this time not only pushing the extruder into the corners, but also slightly lifting it. After doing this and fastening everything in a strategic fashion to get everything aligned properly, now there’s enough tension on the short belt, and the homing succeeds. And the axis check has also successfully run through. I’ll be replacing the short belt though, to be sure, it doesn’t happen again soon, because it got lengthened.