This error can appear during the Self-test or the XYZ-calibration when the firmware detects a shorter distance of axis travel than expected. The exact length of each axis is stored in the printer's memory. Depending on where it is triggered, it will name the running process (Selftest, XYZ calibration, etc.) and will state the message "Please check: Axis length" on the display and the letter for the problematic axis, in the bottom right ("X/Y/Z").
Most often it indicates an issue on the hardware level, and as it is the same method used for Crash detection on the Original Prusa MK3/MK3S/MK3S+ printers, the issue should initially be troubleshot the same way. Check that belts and pulleys are installed as is shown in the Assembly manual, and make sure the axis can move smoothly along the full length of the axis, in both directions. Nothing should be loose. Make sure the pulley set-screw is secured on the flat side of the motor shaft. See Crash detection for more information.
|Correct X-axis pulley orientation and position.
|Correct Y-axis pulley orientation and position.
Take a look at the extruder from behind, make sure that the cable bundle with the textile sleeve (or spiral wrap) isn't too wide, and touching the frame of the printer before the extruder can move to the initial position.
Make sure the zip ties are situated exactly like instructed in the assembly manual. You want the top zip ties on the very top (green arrows) or cleanly against the printed cable-holder. Zip ties must not interfere with either end of the axis Einsy-door or the PSU. Move the extruder to each end and check if the zip ties are touching either end.
The bottom zip ties holding the heater and thermistor cable must also be positioned so they do not interfere with the ends of the axis (purple arrows), but also do not touch the Heatbed-cable-cover. Make sure they are cut flush with the head of the zip tie.
The main end-stop of the Z-axis is the P.I.N.D.A. sensor, so you should first make sure this is not set too high or not working correctly. See P.I.N.D.A./SuperPINDA Sensor testing for more information.
However, more often there is an issue with your assembly, which interferes with the distance traveled. You need to look over your assembly and pay extra attention to the orientation of the Trapezoid nuts (Z lead screw). A section of its threads should be inside the printed part, not protruding up from the printed part. The highest point should be the M3 bolts securing the Trapezoid nuts. Also, if the error occurs during calibration, make sure nothing is touching the bed, i.e. a zip-tie on the bundle of cables.
|Correct orientation of the Trapezoid nut (Z lead screw).
Another possible cause is that you have loaded an incorrect version of the firmware. For instance, the extruder for the Original Prusa MK3 and MK3S/MK3S+ are of different size, and therefore have a different length of expected travel. Always make sure you are using the correct firmware.
For more info on issues with the Z-axis, please see Failing Z calibration (MK3S/MK2.5S).
Try to lubricate the smooth rods, as instructed in Maintenance tips. The Original Prusa MK3S/MK3S+ is supplied with a tube of lubricant.