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Selftest Failed (MK4)

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Last updated 8 months ago
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When you first turn on your Original Prusa MK4 printer, it will start a selftest to verify the most common components of the printer.

Make sure that your Firmware is always up to date in our Downloads page

What it tests

The printer goes through the following procedures:

  • Hotend fan check
  • Print fan check
  • Switched fans check
  • Loadcell test
  • X-axis check
  • Y-axis check
  • Z-axis check
  • Nozzle heater check
  • Heatbed heater check
  • Gearbox alignment
  • Filament sensor test

The Loadcell test, Gearbox alignment, and Filament sensor test need user input, so stay near the printer for these parts of the selftest.

After the selftest, you can check the results at any time in the printer menu in Control -> Diagnostics -> Show Selftest Result.



In case one of the fans fails the selftest, verify if there is nothing blocking the rotation of the fans, as debris might get between the blades. Bigger parts can be cleared with tweezers, and dust can be cleared with a computer cleaner spray.

Hold the fan with your hand while using the compressed air. Otherwise, you might damage the blades.

In case the fan is clean and the issue persists, open the LoveBoard cover and the fan door, and check the connection print fan or the heatsink fan. Make sure that they are in the correct slot.


During the Loadcell test, you will be asked to tap the nozzle at a specific time. In case you do not tap the nozzle, the printer will restart the loadcell test.

If the Loadcell is being triggered too soon, make sure that you are not touching any other part of the extruder other than the nozzle. 

This test can be skipped, but skipping it can cause the nozzle to damage your steel sheet and heatbed.

Only touch the nozzle when the selftest requires you to. Touching it at other times may cause burns.

X/Y Axes

In the case of a failure in the X or Y axis, check the following:

    • Make sure that nothing is blocking the axis path.
    • Turn off the printer and move the axis by hand. It should be moved easily.
    • Check the belt pulleys on the Y or X motor shaft. One of the set screws must be firmly tightened over the flat part of the motor shaft, there should be a small gap between the motor and the pulley, and they need to be in the correct orientation and position (shown below).
    • Check if the belts aren't extremely loose, as they might skip on the teeth of the belt pulley.
    • Try to clean, then apply lubricant to the smooth rods.
    • Check if the M3x30 screw that holds the fan door is firmly tightened.
Correct X-axis pulley orientation and position.Correct X-axis pulley orientation and position.
Correct Y-axis pulley orientation and position.Correct Y-axis pulley orientation and position.
The fan door screw must be tightened fully


The Z-axis check will not be made if the Loadcell test is skipped.

In case the Z-axis test does not pass, check the direction of the trapezoid nut. Make sure that the hotend is properly positioned in the Nextruder.

Nozzle/heatbed heater

After the filament sensor test, the selftest will heat up the nozzle and the heatbed to test them. The steel sheet needs to be on the heatbed for this test.

In case you get any errors, scan the QR code shown on the screen, or check our Knowledgebase.

In case you have a Prusa Nextruder Silicone, go in the printer menu to Settings -> Hardware -> Nextruder silicon sock, and toggle it ON.

Filament sensor

If the filament sensor does not detect the filament during the selftest, you can check the LoveBoard to see if the cable is properly connected. Also, make sure that there is no debris in the Prusa ball holder.

It is possible to skip this part of the selftest, but in this case, the features that depend on the filament sensor might not work.

If you open the filament sensor mechanism in the hotend, note that there are very small parts in it that may be lost!
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