Summary

• New features

  • Automatic chamber vent opening/closing [C1]
  • Auto-retract improvements
  • Auto-retract disable option

• Fixes

  • Fixed XL toolcrashes [XL]
  • Fixed nozzle heating for unload when auto-retracted
  • Fixed XL enclosure not reporting temperature (#4800, #4829) [XL]
  • Fixed sensor info showing only zeroes (#4808)
  • Fixed stack overflow when downloading from Prusa Connect and opening Prusa Link at the same time
  • Fixed getting stuck in tool mapping after homing calibration gets recommended (#4799)
  • Fixed BSOD on M501 (#4912)
  • Fixed absorbing heat status showing wrong value (#4754)
  • Fixed BSOD when opening the door during nozzle cleaning [C1]

• Known issues [XL, MK4S, MK4, MK3.9S, MK3.9]

This is the second release candidate for the upcoming firmware 6.4.0 for the Prusa CORE One, MK4S, MK4, MK3.9S, MK3.9, MK3.5S, MK3.5, MINI & XL. It reunifies the firmware development branch after the temporary split during the CORE One launch, where 6.3.x was released exclusively for CORE One and other printers remained on 6.2.x – along with bringing new features, improvements, and fixes across all platforms.

For a complete overview of features introduced in firmware 6.4.0, users can refer to the 6.4.0-alpha release notes and 6.4.0-RC release notes published earlier.

Automatic chamber vent opening/closing [C1]

The automatic chamber vent mechanism introduced with the CORE One L is now also available on the CORE One. After updating, the feature can be enabled via Settings → Hardware → Chamber Vent Control by selecting the Auto option.

A Manual mode is also available, which keeps the vents closed by default and only shows an informational reminder before each print.

Vents can be controlled manually using the new M870 G-code ("M870 O" = open, "M870 C" = close), although we do not recommend including these commands in slicer profiles, as the printer handles vent control automatically.

The printed parts required to retrofit this feature on existing CORE One printers will be published on Printables in the coming days. A link will be added here once available.

Auto-retract improvements

The auto-retract system has been improved across the board, with particularly noticeable benefits for multi-tool prints on the XL. It now respects slicer-issued retractions and avoids adding redundant movements.

On the MK4, the print head now parks at a higher Z-height after print completion to make it easier to remove any ooze left by the auto-retract ramming process.

Auto-retract disable option

Auto-retract can now be disabled globally in the Settings menu, although this is not recommended for standard use.

It is also possible to disable auto-retract per filament type. In the filament detail menu, the former "Flexible" option has been renamed to "Auto Retract", with its behavior adjusted accordingly.

Fixed XL toolcrashes [XL]

Toolcrashes on multi-tool XL printers introduced in the alpha firmware have been fixed. The 6.4.0 firmware should now be fully functional on the XL printers.

Fixed nozzle heating for unload when auto-retracted

Printers no longer heat the nozzle for unload if the filament is auto-retracted.

Known issues

Prusa XL – Tool recovery: In some cases, triggering a tool recovery causes the print to stop responding. The nozzle and bed remain heated, but the print does not resume and cannot be recovered.

MK4S, MK4, MK3.9S, MK3.9 – The filament sensor only reacts to the first runout event during a print. Subsequent runouts are not detected until the print is restarted.

Both issues will be addressed in final release version of 6.4.0.

Full Changelog: v6.4.0-RC...v6.4.0-RC2