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Firmware 4.0.3-RC1 for the Original Prusa MINI  

Jakub Dolezal
Firmware 4.0.3-RC1 for the Original Prusa MINI

Hi guys,

this is a testing release for the experienced users 😎 

What is new?

  • Fixed mounting of USB drives
  • Support for a new encoder
  • Mesh Bed Levelling fixed
  • Fixed loaded filament information
  • Active heating for cancelled prints fixed
  • Automatic filament sensor detection
  • Improved filament changes (M600)
  • First layer calibration tweak
  • Optimized on-screen messages
  • Fixed random moves after a USB drive is removed
  • Flow adjustment no longer ignored
  • Repeatable purge
  • Crash Dump feature
  • Classic Jerk fix
  • USB CDC fix
  • More detailed error screen
  • Earlier filament load

Detailed description

This is a bugfix release, which takes care of several known issues. Fixed mounting of USB drivers and support for the new LCD encoder are among the main features of this release. The developers have also changed the compiling system to a newer, more robust one.

Fixed mounting of USB drives

During the internal testing, it was discovered that some USB drives aren't properly detected if they are inserted after the printer already finished booting into the system. Once the mounting procedure failed, the system was unable to mount any other flash drive. In cooperation with the chipset producer, we have updated the USB core libraries and the mounting issues are now solved.

Support for a new encoder

As explained in this article, we have encountered a slightly larger than the average fail rate of the LCD encoder, which is used to navigate through the user interface. The new revision of the LCD board is now equipped with the same encoder used on the MK3S. The old and new encoders differ, which means the firmware can now automatically detect the type of the encoder on the startup, so there is no need for user intervention.

Mesh Bed Levelling fixed

In the previous release, when mesh bed levelling procedure failed (e.g. due to an incorrectly placed steel sheet) the printer didn't show the correct error message. In the current release, the printer will display a “Mesh bed levelling failed” message on the screen with two options to either “Retry” or “Cancel” the procedure.

Fixed loaded filament information

This feature introduced with the MINI is used to store information about the currently loaded filament in order to provide faster unloading times. However, in some scenarios, the printer didn't save the value correctly - this is now fixed. Also, in certain situations, the printer is no longer asking for the preheat temperature. For example, when the next print is going to use PLA and PLA filament is already loaded.

Active heating for a cancelled print fixed

If the print was first paused and then cancelled, the printer partly ignored the last action and continued with heating up both hotend and heatbed. This now fixed and the printed cools down.

Automatic filament sensor detection

The MINI is now able to detect not only that the filament was loaded through the sensor, but also that the sensor is connected to the printer.

Improved filament changes (M600)

The code responsible for the execution of the G-code M600 (filament change) was improved to provide a more stable and reliable behavior.

First layer calibration tweak

The end of the calibration pattern (the rectangular shape) was adjusted. The printer lays down the perimeters first and then proceeds to the infill.

Optimized on-screen messages

The management system responsible for displaying the messages on the printer's screen was updated to prevent overlapping messages, which sometimes caused the screen to flicker.

Fixed random moves after the USB drive is removed

If the USB drive was removed after the print had finished, the printer performed several “random” moves in an attempt to perform a re-homing procedure., This is now fixed.

Flow adjustment no longer ignored

Until now any adjustment to the “Flow” parameter in the printer's menu was ignored. This bug is fixed in this release.

Repeatable purge

Due to the technical difficulties, the printer was unable to repeat the purge procedure, if the filament wasn't loaded all the way through the print head to the nozzle. The current release fixes this issue and offers multiple consecutive purges to load the filament properly.

Crash Dump feature

The advanced 32bit SoC running the Buddy board enables logging the system events. In the case of a system crash, the information is written in the SPI flash. After the printer is restarted, users can download the Crash Dump file onto a USB drive and provide it to the support team/developers for debugging purposes.

To download the Crash Dump file, insert a USB drive into the printer first and then head to “Settings -> Save Crash Dump”.

Classic Jerk fix

During the merge of the latest Marlin code to our firmware, the “Junction Deviation” was enabled instead of a “Classic Jerk”. This release fixes it by switching back to the “Classic Jerk”.


The printer is now using the correct VID / PID descriptors and texts including a serial number, while connected over the Micro USB cable.

More detailed error screen

Error screens are undergoing a major redesign. This firmware lays down the foundation for the future version, which will provide a brief description of the issue with a link to an article explaining the solution for the error.

Earlier filament load

This is also a work in progress. The next firmware release will enable the filament to be loaded as soon as the current temperature is 10 °C below the target temperature. This makes the filament loading procedure faster.

Supported printers:

  • Original Prusa MINI

Download link:

How to flash the firmware on MINI:

  1. Copy the firmware file (.bbf) onto the USB drive with FAT32 formatting
  2. Insert the flash drive in the MINI's USB port
  3. Turn the printer ON (or reset it)
  4. During the boot screen hit (press) the knob once
  5. A new screen will appear (if not, repeat the process) 
  6. Confirm you want to flash the firmware
  7. Wait till the firmware is flashed and printer fully boots into the homescreen

Please report any bug here:


As always, we wish you happy printing and look forward to your feedback!

This topic was modified 3 years ago 2 times by Jakub Dolezal
Assembly manuals

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The guy behind Prusa assembly manuals...

Posted : 13/02/2020 9:57 am
Eminent Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI

Does using pre-release firmware void the warranty?  I'd love to test/bug report but also don't want to brick my printer.

Posted : 13/02/2020 2:46 pm
Active Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI

4.0.3 is officially released and verified by PR. No need to brake the "fuse" on the buddy board.



Posted : 13/02/2020 4:55 pm
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI


4.0.3 is NOT released, the Candidate is out, however, it is known buggy.

Posted : 13/02/2020 8:48 pm
New Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI

"Feature Request" Multiple sheets choice from menu (PEI and TXT)

Posted : 16/02/2020 7:38 pm
Trusted Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI

With this firmware (and earlier), is the only way to print to the printer through saving to a USB drive? Seems that way.


  • No support for connecting via the micro USB?
  • No support for ethernet? (This is what I really want, and is how I use my SL1)

I got my Mini today, and was very disappointed to see that neither of those options worked. Doesn't seem like this firmware fixed that. In fact, 4.0.2 has the ethernet icon lit up, but this 4.0.3 turns it off.

Any idea when those will be in?

Posted : 18/02/2020 1:45 am
Trusted Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI


This should be the right place for requests like that...

It’s better to give than to receive. Especially advice.

Posted : 18/02/2020 7:46 am
New Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI


Thanks, it's already done. 



Posted : 18/02/2020 9:54 am
Noble Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI


Several Mini users report that Octoprint works successfully.

Posted : 18/02/2020 12:11 pm
Noble Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI


My understanding is that with the breakout on the controller board in place, only signed firmware can be written. An attempt to write unsigned firmware will just fail, hopefully gracefully.

Posted : 18/02/2020 12:27 pm
Honorable Member
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI

Does anyone know of the implications of Junction Deviation vs Classic Jerk fix.. Does this change affect the quality in any way?

I am currently struggling with a problem on my Mini and would rather not use pre-release firmware.. But if there are any quality benifits, I would like to be on the latest.

Posted : 18/02/2020 3:23 pm
RE: Firmware 4.0.3-RC1 for the Original Prusa MINI


successfully is not exactly true. Can you use it. "yes" can you use it successfully not yet. it has a tendency to air print due to weird retraction issues.

Posted : 18/02/2020 3:29 pm