Z axis hits end stop, the goes below bed

Hi guys, printer was working fine, (ramps 1.4 with mechanical end stops and marlin firmware)

All of a sudden, when I click print, the axis will return to their endstops, including z. The z axis will then raise itself, stutter as though they are fighting something, and then lower itself again approximately 2mm below the bed. I have not changed any settings, there is no z offset set, and I have tried old g-code that I know use to not have this problem.

Any suggestions?

Cheers,

There's instructions in ramps to, I believe, raise the hot end while it and the bed come to the correct temperature, then lower it again. The speed for this is HOMING_FEEDRATE in Configuration.h. If you're getting a bit more resistance in your Z travel, HOMING_FEEDRATE might be too high for your motors to handle, resulting in them skipping steps. Try lowering HOMING_FEEDRATE.

Source: had the same problem and had to lower from 4*60 to 3*60 to get it to move properly.

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Thanks, turned out the pots just weren't cranked high enough!