Fixing Prusa MK4 Y-Axis Homing and Self-Test Failures

The Prusa MK4's advanced sensor systems are fantastic for preventing damage and diagnosing problems - until they're not. Unexpected self-test failures and Y-axis homing issues can bring production to a halt, often with little explanation of what went wrong. While developing auto ejection for MK4, we discovered that these seemingly random failures actually follow distinct patterns, and better yet, they're completely fixable. In this blog we’ll go through the issue itself, how we discovered it, and its fixes.

Contents:

The Problem
The Solutions
The Result

The Problem

The MK4's self-test and diagnostic systems are designed to catch issues quickly, and therefore are quite sensitive. While this works wonderfully for catching genuine issues, it can also end up overreacting and actually causing issues. In some cases, you might find your printer constantly recalibrating, with the Y-axis repeatedly bashing against the back of the printer during what should be a simple homing procedure.

Our testing revealed several specific situations where Y-axis homing would fail unexpectedly. These failures occurred consistently across both factory-assembled and manually assembled MK4 printers, indicating this wasn't related to assembly quality, but of ongoing maintenance. The issues would appear when:

Switching the build plate (e.g., from the default satin sheet to the official textured PEI surface)
Attempting to home with a completed part still on the bed
Homing with heavier parts on the bed
Enabling input shaping during certain parts of the print
Printers were tilted for auto part ejection
Printers had not undergone maintenance for after extended periods of use

The Solutions

Through extensive testing, we've developed three reliable approaches to resolving these Y-axis issues, starting with mechanical adjustments and moving to sensitivity tuning only when needed.

Solution #1: Mechanical Adjustments

For most printers, these basic mechanical fixes resolve the issue:

Re-tighten all the screws on the X and Y axis, including:
- Idler screws
- Linear rod screws
- Y axis motor mount screws
- Y axis bearing holders
Clean and re-lubricate the Y axis rods
Re-tension the Y belt (note: you may need significantly more tension than the tuner recommends)
Confirm your Y axis bearings are properly seated. You may have to push them closer to the centre of the bed than their guides recommend.

Important note: Even on factory-assembled machines, screws will naturally come loose over time. Regular maintenance checks are essential for preventing issues before they occur.

Solution #2: Motor Alignment

After extended use, we discovered the Y axis motor can begin to rotate in its mount. This rotation causes the belt to twist, which can falsely trigger the Y axis homing sensor. If you're experiencing consistent issues:

Check the motor's alignment
Straighten if needed
Firmly tighten the mounting screws

Solution #3: Sensitivity Adjustments

If mechanical fixes don't fully resolve the issue, adjust the printer's settings:

For Alternative Bed Surfaces: When using different print surfaces, including the official textured PEI sheet, you'll need to:
- Use the following G-code to tell the machine to expect additional weight before homing
  M74 W250; set bed weight to 250g
- Recalibrate the axis after changing the bed using the MK4's built-in calibration menu
Manually increase or decrease the homing/crash detection sensitivity in your start gcode:
```
M914 Y-5; set Y axis sensitivity to -5
```
Prusa self calibrates this value to -3 to -5. Setting this value more negative will increase the homing sensitivity, and will help home the Y axis more consistently if the motors are providing weak feedback (constant bashing of the Y axis).

Important: This should be your last resort after trying all mechanical fixes.

This adjustment is particularly important when there's extra weight on the bed during homing, such as during automated part removal. The printer's default settings aren't configured to expect this additional weight, so temporarily reducing the motor sensitivity prevents false triggers while maintaining protection against actual crashes.

The Result

By following these solutions in order, you can maintain the MK4's protective features while preventing false positives from the crash detection system. Whether you're using different build surfaces or dealing with parts on the bed during homing, these adjustments help ensure reliable operation without compromising the printer's safety systems.

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