6.3.4-RC Firmware for Prusa CORE One

Summary

• Smarter heat absorption logic
• Retuned heatbed PID for thermal stability
• Automatic display refresh rate fallback
• Additional updates, improvements, and fixes

This is the release candidate for the upcoming firmware 6.3.4 for the Prusa CORE One, addressing several improvements and bug fixes.
This release focuses on enhancing reliability and fine-tuning the user experience based on recent feedback and internal testing. Users can expect smoother operation, better thermal stability, and quality-of-life improvements across multiple areas of the firmware.

Smarter heat absorption logic

The heat absorption logic has been significantly improved to make your printer smarter — and your workflow faster. The firmware now continuously tracks recent heatbed activity and dynamically adjusts the required stabilization time before printing. If the bed has already been heated, such as when printing multiple objects in succession, switching materials, or using preheat, the usual heat absorption delay is automatically shortened — or even skipped entirely.
This leads to a noticeably faster transition into printing, especially during reprints or tightly timed jobs, helping you save time, stay productive, and get the most out of every print session.

Retuned heatbed PID for thermal stability

PID values for the CORE One heatbed have been fine-tuned to provide more stable temperature regulation and improved print consistency.

Automatic display refresh rate fallback

If repeated display anomalies (such as flickering) are detected, the printer now shows a prompt offering to switch to a lower refresh rate. To ensure the dialog remains readable even if the display is unstable, the system temporarily applies the lower refresh rate before displaying the prompt. If the user selects "No," the original (higher) refresh rate is restored.
The display refresh rate can also be adjusted manually at any time in Settings → Hardware → Display Refresh Speed.

Additional updates, improvements, and fixes

Progress reporting during filament auto-retract

The progress bar is now updated during the auto-retract phase of filament load or change, instead of remaining stuck at 99% in the "Loading" phase.

Option to disable grille check warnings

The ventilation grille warning now includes a “Disable” button. The warning can also be managed under Settings → Hardware → Check Ventilation Grilles.

N/A entries removed from print carousel

Print screen carousel now omits entries marked as “N/A” to reduce clutter and improve readability.

Increased retraction after filament change

Retraction during filament unparking has been increased from 2 mm to 4 mm to minimize oozing.

Extended post-print filtration limit

The maximum setting for post-print air filtration has been extended from the previous limit to 90 minutes.

Phase stepping calibration option

A calibration option is now available to fine-tune phase stepping on the CORE One. The feature requires a PRUSA Accelerometer Set and is intended for advanced users who enjoy exploring deeper tuning options and experimenting with motor behavior.

Chamber lights stay on with the camera enabled

When the integrated camera is active, chamber light dimming is now disabled by default to maintain optimal visibility. This behavior can still be manually changed in the settings.

Nozzle no longer heats unnecessarily on unload

The nozzle will no longer be heated unnecessarily during filament unload if the filament has already been auto-retracted and only needs to be deretracted for gear clearance.

Nozzle now cools properly after unloading

After filament unload, the nozzle now properly begins the cooldown process.

Corrected door sensor calibration instructions

Corrected the wording in the door sensor calibration wizard. The prompt now properly instructs the user to loosen the tensioning screw, not tighten it.

6.3.3 Firmware for Prusa CORE One

Summary

• Added Ukrainian translation
• Door sensor now disabled by default
• Configurable PTFE Tube Length for MMU3
• Timeout added for ventilation grille prompt
• Improved G-Code support for chambers (#4479, #4484)
• Fixed BSOD when preheating with "Custom" filament
• False “Fan Not Spinning” errors on DIY filtration should be resolved (#4593)
• Fixed footer not updating after selection (#4345, #4166, #4560)
• Increased max supported nozzle diameter (#4565, #4553)

Ukrainian Translation

The printer interface is now available in Ukrainian. Вдалого друку! (Happy printing!)

Door Sensor Disabled by Default

The door sensor emergency stop is now disabled by default on new Core ONE units. You can enable it in the settings.
Note: The sensor must remain connected to the xBuddy board to identify the printer model.

Configurable PTFE Tube Length for MMU3

You can now set the PTFE tube length between the extruder and the MMU under Settings → Hardware → MMU.

Ventilation Grille Prompt Timeout

Prompts to open or close the ventilation grilles on Core ONE now automatically dismiss after 60 seconds.

Improved G-Code Chamber Support (#4479, #4484)

Chamber temperature is now included in periodic serial reports and M115 capability info. Thanks to @bkerler and our team for the contribution.

Fixed BSOD when preheating with "Custom" filament

Resolved a BSOD caused by long translated strings when preheating “Custom” filament in some languages.

False “Fan Not Spinning” errors on DIY filtration should be resolved (#4593)

Added signal filtering to reduce false fan errors on DIY filtration setups using MK4 fans with longer cables. This should resolve tachometer signal issues.

Fixed footer not updating after selection (#4345, #4166, #4560)

Fixed a bug where the footer didn’t update correctly after selecting “none.” Thanks to @bkerler for the fix!

Increased max supported nozzle diameter (#4565, #4553)

Nozzle size can now be set up to 1.8 mm. Thanks to @bkerler!

Known Bug

Running a selftest with damaged or unplugged fans shows a “Fan is not spinning” error instead of a failed selftest.

Full Changelog: v6.3.2...v6.3.3

6.2.5 Firmware for Original Prusa XL

Summary

• Fixed hangs during fast consecutive toolchanges

This is the stable release of firmware 6.2.5 for Original Prusa XL.

Fixed hangs during fast consecutive toolchanges

We addressed an issue where the printer could hang for several seconds during quick, consecutive tool changes.

The problem was traced to the print fan going from full power to off upon tool parking. This destabilized the nozzle PID regulator, which in turn made the M109 “Wait for hotend temperature” G-code wait until the temperature stabilized again if the tool was picked too soon after being parked.

We have calibrated the PID regulator so that it would not destabilize due to print fan speed changes.

6.2.4 Firmware for Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL

Summary

• Reduced USB errors
• CORE One printer detection
• Fixed returning to wrong Z position after pause
• New print fan type [XL]
• Improved extruder error detection [XL]

This is the stable release of firmware 6.2.4 for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL.

Reduced USB errors

We fixed a rare condition that could occasionally trigger a “USB Error” pause during printing.

CORE One printer detection

The firmware brings a new bootloader version 2.4.1, which is able to recognize MK4S from CORE One (based on whether the door sensor is connected).

NOTE: Before you convert your MK4S to CORE One you need to update the FW to 6.2.4. If xBuddy is already connected in CORE One and the door sensor is attached, the bootloader refuses to update. We will remind you of this in the MK4S to CORE One Conversion Guide.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.
This was already fixed in 6.2.3, but 6.2.3 was only released for MK4 and MK3.5. 6.2.4 brings this fix to the XL as well.

New print fan type [XL]

We have extended our list of suppliers for print fans on the XL. The new fans have slightly different properties. Since the printer cannot automatically detect which fan is installed, we have added a settings option (Settings -> Tools Settings -> Print Type Fan) for selecting the correct fan type the printer is equipped with.

Improved extruder error detection [XL]

Extruder errors (extruder not spinning) should no longer trigger for parked tools.

6.3.2 Firmware for Prusa CORE One

Summary

• Adjusted RPM range check for the Print Fan test

This is the stable release of firmware 6.3.2 for the Prusa CORE One, addressing issues with certain print fans.

Adjusted RPM range check for the Print Fan test

The lower threshold of acceptable fan speed during the Print Fan test has been reduced from 5130 rpm to 4800 rpm. This change improves compatibility with fans operating near the lower end of their specified speed range.

6.2.4-RC1 Firmware for Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL

Summary

• Reduced USB errors
• CORE One printer detection
• Fixed returning to wrong Z position after pause
• New print fan type [XL]
• Improved extruder error detection [XL]

This is a public release of the 6.2.4-RC firmware for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S, MINI & XL.

This is a release candidate intended for experienced users. We’ve not been translating changed/newly added texts yet. The features in this firmware are still under development and may not perform as expected. Should you encounter any bugs or issues with the printer’s performance, please report them via GitHub or in our forums.

Reduced USB errors

We fixed a rare condition that could occasionally trigger a “USB Error” pause during printing.

CORE One printer detection

The firmware brings a new bootloader version 2.4.1, which is able to recognize MK4S from CORE One (based on whether the door sensor is connected).

NOTE: Before you convert your MK4S to CORE One you need to update the FW to 6.2.4. If xBuddy is already connected in CORE One and the door sensor is attached, the bootloader refuses to update. We will remind you of this in the MK4S to CORE One Conversion Guide.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.
This was already fixed in 6.2.3, but 6.2.3 was only released for MK4 and MK3.5. 6.2.4 brings this fix to the XL as well.

New print fan type [XL]

We have extended our list of suppliers for print fans on the XL. The new fans have slightly different properties. Since the printer cannot automatically detect which fan is installed, we have added a settings option (Settings -> Tools Settings -> Print Type Fan) for selecting the correct fan type the printer is equipped with.

Improved extruder error detection [XL]

Extruder errors (extruder not spinning) should no longer trigger for parked tools.

6.3.1 Firmware for Prusa CORE One

Summary

• Filtration support
• MMU support
• Automatic filament retraction
• Configurable chamber cooling maximum power
• Set chamber target temperature in preheat
• Chamber fan spin check
• Ventilation grille prompt
• Door sensor calibration wizard
• Filament change confirmation prompt
• Option to disable filament sensor during filament change
• Added support for the “M30 Delete file” gcode [Public PR]
• Increased “failing to reach chamber temperature” timeout
• Increased maximum bed temperature
• Park printhead in the back after print
• MMU: Better nozzle compatibility check
• Fixed: Restart on door open during pause
• Fixed: Incorrect Z position when unpausing
• Reduced USB errors
• MMU: Fixed power panic during change

This is the stable release of firmware 6.3.1 for the Prusa CORE One. It includes support for CORE One add-ons (MMU3, advanced filtration), improvements to existing features, and fixes for known issues.

We have also ported the fixes and features introduced in firmware versions 6.2.2 and 6.2.3, except for the layer skipping fix, which was not included in 6.3.0.

Filtration support

We have implemented support for the Advanced Filtration System addon. When printing with filaments marked as "Requires filtration", the system automatically activates and manages the filtration fan during and after the print.
Users can now configure fan power and post-print filtration duration directly in Settings → Chamber Filtration or via the Tune menu during printing.

We’ve also added support for DIY setups without a dedicated filtration fan. In this mode, the existing cooling fans handle both cooling and filtration. This option can be enabled under Settings → Chamber Filtration → Chamber Filtration → DIY.

• Minimum Power - sets the minimum power level for the additional filtration fan. The printer will adjust the fan speed based on the chamber temperature control system, up to the “Chamber Fans Limit” setting.
  Note: A minimum of 40% is needed for effective filtration of the entire chamber.
• Post-print Filtration - toggles chamber filtration after the print is finished (strongly recommended).
• Post-print Duration - sets the duration of additional filtration after the print is complete.
• Post-print Power - adjusts the power level of post-print filtration to suit your needs.
  Note: A minimum of 40% is needed for effective filtration of the entire chamber

In the Control → Temperature menu and the Tune menu (during printing), you can manually set the power of the filtration fan, just like you would for the chamber fans. It’s also possible to manually use both the chamber and filtration fans simultaneously.

Note: When enabled, the Advanced Filtration System also takes over chamber temperature control and disables the cooling fans (with the exception of critical overheat situations).

MMU support

The CORE One now supports the MMU3 (Multi-Material Unit), with the same integration and behavior as on the MK3.5 and MK4. This allows for multi-material printing directly on the CORE One with all standard MMU3 features.

Automatic filament retraction

We’ve introduced automatic filament retraction at the end of a print, with reinsertion just before the first extrusion. This brings several benefits:
No filament remains in the nozzle during mesh bed leveling, helping prevent filament specs from being left on the print bed.
The nozzle no longer needs to be heated for filament unload.

Note: This is a pilot implementation of the procedure. If it proves effective, we plan to extend it to other printers as well.

_Note: Auto-retraction is not performed for flexible filaments. To support this, we’ve added an "Is flexible" parameter for filaments, configurable via the GUI (for user presets) or using the M865 G-code.

Configurable chamber cooling maximum power

The maximum power of the chamber cooling fans was previously hard-capped at 40%, as higher speeds were considered too noisy and unnecessary. This limit is now user-configurable.

Set chamber target temperature in preheat

During preheat, the chamber target temperature is now automatically set according to the filament profile’s Nominal Chamber Temperature value.

Chamber fan spin check

We now check that the chamber fans are spinning when they should and show a warning (and pause the print) when they don’t.

Ventilation grille prompt

When printing materials with a target chamber temperature below 45 °C, the printer now prompts the user to open the top ventilation grille. For filaments requiring higher chamber temperatures, the user is advised to close the vent.

Note: This prompt is shown only once and is skipped for consecutive prints with the same configuration.

Door sensor calibration wizard

A new calibration wizard has been added to help properly set up the door sensor, especially for CORE One kits.

The wizard guides the user through checking these door positions:

• Fully closed
• Fully open
• Partially open (simulating fingers between the door and frame)

If any of these states do not match the expected behavior, the user is prompted to adjust the sensor by loosening or tightening its position.

Filament change confirmation prompt

The first item in the Tune menu is Change Filament. Accidental double-clicks on the Tune button could unintentionally trigger a filament change during a print.
To prevent this, users are now prompted to confirm the action before it proceeds.

Option to disable filament sensor during filament change

We’ve added a help button to screens where users might get stuck during the filament change process due to a triggered filament sensor. This button opens a screen with instructions on how to clean the sensor, along with an option to temporarily disable it.

Note: Please note this feature cannot be backported to the 6.2 firmware. Since firmware 6.3.1 is being released for the CORE One only, other printers will receive this feature in a future update.

Added support for the “M30 Delete file” gcode [Public PR]

Thanks to @bkerler, the firmware now supports the M30 G-code for deleting files directly from the printer. Nice!
PR #4429

Increased “failing to reach chamber temperature” timeout

The "Failing to reach chamber temperature" warning is now triggered after 30 minutes instead of the previous 10 minutes, giving the system more time to stabilize before alerting the user.

Increased maximum bed temperature

The maximum bed temperature has been increased from 115 °C to 120 °C. This aligns it with the MK4S, which uses the same heatbed. The previous limit was lower due to an oversight.

Park printhead in the back after print

Based on user feedback, the printer now parks the printhead in the back after a print and during pauses to avoid obstructing the Buddy3D Camera's view.

For filament changes, the printhead still parks in the front for easier access. Homing also remains at the front due to the sheet detection process.

MMU: Better nozzle compatibility check

When printing MMU G-codes, the firmware now warns users if a file sliced for a standard nozzle is started with a high-flow (HF) nozzle. HF nozzles require more purging in the wipe tower, and using a non-HF G-code can lead to unreliable results.

Bug fixes

Restart on door open during pause (C1)

Opening the door during a pause no longer causes the printer to restart. While it still interrupts the pause sequence for safety reasons, it should now behave more gracefully. We still recommend opening the door only after the printer has fully stopped.

Incorrect Z position when unpausing

Unpausing a print no longer skips Mesh Bed Leveling (MBL) corrections during the unpark move. This fix is included in 6.3.1 for CORE One and also in 6.2.3 for other printers.

Reduced USB errors

We fixed a race condition that could occasionally trigger a “USB Error” pause during printing.

MMU: Fixed power panic during change

We have resolved an issue where Power Panic would fail to properly resume if triggered during an MMU filament change.

6.2.3 Firmware for Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S

Summary

• Hotfix for the MMU overcurrent
• Fixed returning to wrong Z position after pause

This is a stable release of firmware version 6.2.3 firmware for the Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5 & MK3.5S.

Hotfix for the MMU overcurrent

A recent supply change of a few electrical components has caused issues during the MMU power-on procedure due to the new components having different tolerances.

We have adjusted the MMU power-on procedure to accommodate this change. The current solution is a hotfix that introduces a small (~1s) delay when powering on the MMU. A more permanent solution will be released soon.

We would like to thank you, the community, for your efforts regarding this issue. Your observations and discoveries have made it much easier for us to locate the problem and focus more on how we could fix it.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.

6.2.3-RC Firmware for Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S

Summary

• Hotfix for the MMU overcurrent
• Fixed returning to wrong Z position after pause

This is a public release of the 6.2.3-RC firmware for the Original Prusa MK4/S, MK3.9/S, MK3.5/S.

This is a release candidate intended for experienced users. We’ve not been translating changed/newly added texts yet. The features in this firmware are still under development and may not perform as expected. Should you encounter any bugs or issues with the printer’s performance, please report them via GitHub or in our forums.

Please note that this is a testing firmware developed primarily on stock unmodified printers. If you are running any 3rd party upgrade, exercise caution and let us know in case of any issues.

Hotfix for the MMU overcurrent

A recent supply change of a few electrical components has caused issues during the MMU power-on procedure due to the new components having different tolerances.

We have adjusted the MMU power-on procedure to accommodate this change. The current solution is a hotfix that introduces a small (~1s) delay when powering on the MMU. A more permanent solution will be released soon.

We would like to thank you, the community, for your efforts regarding this issue. Your observations and discoveries have made it much easier for us to locate the problem and focus more on how we could fix it.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.

6.2.3-alpha Firmware for Original Prusa MK4, MK4S, MK3.9, MK3.9S, MK3.5, MK3.5S

Summary

• Hotfix for the MMU overcurrent
• Fixed returning to wrong Z position after pause

This is a public release of the 6.2.3-alpha1 firmware for the Original Prusa MK4/S, MK3.9/S, MK3.5/S.

This is an alpha release intended for experienced users. We’ve not been doing translations for changed/newly added texts yet. The features in this firmware are still under development and may not perform as expected. Should you encounter any bugs or issues with the printer’s performance, please report them via GitHub or in our forums.

Hotfix for the MMU overcurrent

A recent supply change of a few electrical components has caused issues during the MMU power-on procedure due to the new components having different tolerances.

We have adjusted the MMU power-on procedure to accommodate this change. The current solution is a hotfix that introduces a small (~1s) delay when powering on the MMU. A more permanent solution will be released soon.

We would like to thank you, the community, for your efforts regarding this issue. Your observations and discoveries have made it much easier for us to locate the problem and focus more on how we could fix it.

Fixed returning to wrong Z position after pause

When unpausing, the printer was not applying the MBL correction to the unpark move.