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Custom Firmware v1.0 for MK3.5 / MK3.5S – M500 Storage, M851 X/Y, Load/Unload Tuning

INVESTEGATE — Sun, 03 May 2026 15:32:06 +0000

Hi everyone,

I would like to share my first unofficial custom firmware release for the Original Prusa MK3.5 / MK3.5S.

It is based on official Prusa-Firmware-Buddy v6.5.3.

Supported printers:

Original Prusa MK3.5
Original Prusa MK3.5S

Important: This is unofficial custom firmware. It is not made, approved or supported by Prusa Research or Bondtech.

Why I built it

I am proud to share my first modified firmware release.

I use a Bondtech LGX Shortcut setup on my own MK3.5S.

The stock filament load and unload behavior did not always work reliably for me, especially during M600 filament changes while printing.

When the printer is idle, you can work around some issues manually by using Move Axis to load or unload filament. But during an active print, that is not really possible.

In my case, this sometimes caused filament jams, failed filament changes and failed prints when I could not unload or reload the filament properly.

That was the main reason I started modifying the firmware.

Screenshots

Custom firmware splash / boot screen

Experimental menu - custom mod options

Experimental menu - Prusa developer options

Color meaning:

Orange = custom options from this firmware mod
Green = Prusa developer options
Red = planned / future untested options, not used in v1.0

All orange options included in v1.0 were tested thoroughly on my own setup. I did not find any errors during testing.

Main features in v1.0

Direct M851 X/Y controls from the Experimental menu
Reset probe position
Initial insert length
Fast load length
Unload length
Unload cooling retract
Unload ramming scale
Reset load/unload
Persistent saving of custom tuning values via M500
Enable EEPROM save
Reset M500 tuning
Optional print finish melody
M600 / filament change behavior improvements
Safer M500 / M501 / M502 / M503 / M851 behavior

Example for LGX Shortcut

Stock Prusa default:

M851 X23.00 Y5.00

My Bondtech LGX Shortcut target:

M851 X23.00 Y12.00

Important:

Do not copy another printer’s Z / Live-Z value.

Example only:

M851 X23.00 Y12.00 Z

Your Z value must be calibrated on your own printer.

Recommended starting values for LGX / LGX Shortcut

Probe X position: 23
Probe Y position: 12
Initial insert length: 15 mm
Fast load length: 10 mm
Unload length: 45 mm
Unload ramming scale: 10 %
Unload cooling retract: 7 mm
Print finish melody: SURE :D 
Enable EEPROM save: user preference / advanced use

Useful command after flashing to check your EEPROM settings:

M503

Persistent M500 storage

The custom tuning values can be saved persistently via M500.

This means the custom menu values do not have to be re-entered after every reboot.

What is not included in v1.0

v1.0 does not include:

No heater / thermistor profiles
No hotend temperature limit changes
No extruder current menu
No microstep switching menu
No motion limit profile system
No automatic extruder profile selection

I kept v1.0 focused on the features I could properly test on my own printer.

Revo note

Revo should also work since stock firmware 5.2.2, so this mod is not required just for Revo.

Test status

All custom orange menu options included in v1.0 were tested thoroughly on my own setup. I did not find any errors during testing.

Future development / testers wanted

For future versions, I may look into:

Better third-party extruder profiles
Possible hotend / thermistor support
More motion / acceleration related controls
Better UI grouping

But I do not want to add safety-critical features like heater, thermistor or high-temp support without real hardware testing.

If anyone here is running a modified MK3.5 / MK3.5S setup and wants to test, review code or contribute, feedback would be very welcome.

Download

Release v1.0:
https://github.com/INVESTEGATE/Prusa-MK3.5-MK3.5S-Custom-Firmware/releases/tag/v1.0

Firmware file:

mk3.5_firmware_6.5.3_MODv1.0.bbf

Source code:
https://github.com/INVESTEGATE/Prusa-MK3.5-MK3.5S-Custom-Firmware

Full write-up:
https://investegate.de/prusa-mk35-mk35s-custom-firmware-v1-0/

Important warning

This is unofficial custom firmware. Use it at your own risk.

Before flashing:

Save your current M503 output
Save your M851 X/Y/Z values
Save your Live-Z / First Layer Calibration value
Save your M92 E, M906 E and M603 values if relevant

After flashing:

Check M503
Check M851
Check M603
Test filament load
Test filament unload
Test M600 filament change
Run a first layer check
Run a small test print before normal use

For custom firmware flashing on MK3.5 / MK3.5S, the appendix seal on the xBuddy board must be broken first according to Prusa’s official instructions.

Prusa custom firmware / appendix seal guide:
https://help.prusa3d.com/article/flashing-custom-firmware-core-one-l-core-one-mk4-s-mk3-9-s-mk3-5-s_814967

According to Prusa, breaking the appendix seal does not void the printer warranty, but unofficial custom firmware is still used at the user’s own risk.

Thanks to everyone sharing MK3.5 firmware modding information. This release is only a first step, but maybe it is useful for others working with modified MK3.5 / MK3.5S setups.

PETG takes too long…

Steff137 — Sun, 03 May 2026 11:42:52 +0000

Hello everyone!

I find that the start of a print is too long with PETG. I measured it, and the printer takes exactly 16 minutes before starting the print. Do you have any suggestions to improve the startup time?

Thanks for your replies! :)

Where is the Windows Installer?

JOHNE — Sun, 03 May 2026 10:11:00 +0000

Running PrusaSlicer for 1-1/2 years. I am getting a pop-up for an upgrade so I download the zip and it only contains the executable and library files. No windooz installer. I don't remember having to manually install this thing(?)

I googled around to see if there was some announcement about dropping the Windows package. I visited all the usual places (Github, Downloads Page) and they only have the .zip file with distribution files. I googled to see if anybody posted directions on manual installation and the Youtubes all show a download / execute installer process.

What am I missing? I really don't want to have to manually install the thing. I don't do that for the other 4 slicers on my machine.

printer gcode

SdeL — Sun, 03 May 2026 08:45:33 +0000

how can I control the vent grid opening/closure in the printer gcode

Repeated Bed Leveling failure

Steven Clarke — Sun, 03 May 2026 00:39:39 +0000

OK, how do I fix this?

Core One, does 49 points, and says the bed is not flat. I say yes to do the Z-alignment, the bed goes all the way down, then comes back up, and it does the points over again, and still fails. I have turned it off, same problem. I pulled out the print sheet, cleaned it well, cleaned the heating plate well, flipped the print sheet over. None of those fixed it.

Where in the build instructions do I need to go back to to check something?

Power XL Filament Loader for Prusa XL (3-5 filaments)

Ro3Deee — Sat, 02 May 2026 23:00:46 +0000

https://www.printables.com/model/1647918-power-xl-345-filaments-loader-for-prusa-xl

video: https://youtu.be/q9LNOFeEZv0?si=wQU-Q-L4QAMuqE0t

Soluble support material(s) question

chmax — Sat, 02 May 2026 21:08:32 +0000

I 'd like to start soluble materials for the contact layer of support structures and having a hard time sorting out what soluble filament(s) to use with what. I use PLA, PETG, PCBlend, and ASA (there is also a roll of ABS still sitting in the shelf but rarely used).

May I kindly ask those that have experience with this to share it here? Thanks in advance.

Filament settings

Phil — Sat, 02 May 2026 20:38:26 +0000

I am going to print using 3DTech CarbonX ASA+CF. 3DTech gives a nozzle temperature range and a bed temp range.
Prusa Slicer's filament form has Nozzle temp first layer and other layers, same for bed temperature. And chamber nominal and minimal.

Do I use the highest temps given by 3DTech or some mix? How am I to know which to use? Thank you,

Collision detection: bed won't rise

xrayspecs — Sat, 02 May 2026 19:41:57 +0000

The heat bed on my Core One+ won't rise past the screws on the right wall. When I attempt to print I get a Collision Detection error. The initial issue was that the magnetic plate was misaligned and the edge of it collided with those right side screws. I stopped the print and realigned the plate; but ever since then, the bed won't rise past those screws. Ugh. Anyone know what might be wrong, and how it can be fixed?