In most cases, this could be caused by a faulty limit switch. Troubleshoot by the steps below.

1. Remove the glass cover on the left side of the engraver.
2. Remove the metal plate on top of the main board.
3. Check the status of LED5 on the main board.

Under normal circumstances, LED5 will light up when the laser head returns to its origin along the Z-axis, go out, then light up again, and finally go out.

If LED5 is always on, the limit switch Z-axis is faulty or malfunctioning. You can fix it by simply replacing it with a new, identical one.