I'm having an issue with my very first printer, a Monoprice Select Mini V2. After doing about 20 successful prints on it, the Z axis is suddenly acting very odd.

When I 'home' the Z axis and move the nozzle on the X and Y, I can do the paper test on all four corners of the bed without issue - the leveling is near-perfect. But when I start a print job, the nozzle is much lower - by at least 1-2 mm.

This causes the print head to grind against the print bed, which I unfortunately need to replace as it's pretty much destroyed. The nozzle is so much lower at the start of a print job than it is at the home position that it ground a permanent line on the bed. It's trying to go so low, there's enough pressure on the nozzle to not let any filament escape - leaving a bad gouge like I dragged a screwdriver across the print surface. Not good.

It's not the Z-axis limiter switch. I confirmed that is both working and secured tightly to the printer body. When homing the Z axis, I can hear the switch click and the printer stops at that position correctly. It's only when I start a print job that it ends up lower, almost as if it's ignoring the switch.

I also eliminated my slicer software from the equation by printing something I had printed successfully just a couple hours prior - without reslicing or modifying the GCODE file at all. I'm at the point now where I can't print anything that I could before, without having this problem. My heat and speed settings remained untouched.

How can I solve this? The issue popped up just after doing a successful print. What gives? I've heard of the opposite problem (Z-Axis 'too high'), especially after changing nozzles, but not 'too low', and I've never seen it where the print job actually starts lower than the true zero position. Help!