The above article is sorely lacking in detail, especially with regard to what steps to follow when attaching the accelerometer to the print head,
and what will happen after that when you select the calibration of Input Shaper or Phase Stepper.

Here is some of that missing detail.   I hope that Prusa will edit the help article and provide the missing steps.  START WITH THE PRINTER TURNED OFF!

1.  Install the SHORT accelerometer jumper cable between the socket on the main control board that is shown in the help article instructions circled in orange, and the hidden connector inside the WiFi module after you remove the WiFi plastic cover.    Then reinstall the WiFi module and cover, and reinstall the main control board sheet metal cover.
2. Do NOT connect the accelerometer yet.   Turn ON the printer. 
3. Perform Belt Tension check and adjustment if needed.  You do not want to perform Input Shaper and Phase Stepper calibrations until you know the belts are properly tuned.  So check them now.
4. We are now ready to move on to installing the accelerometer.   First: If the build plate and bed are not near the bottom,  go into CONTROL -> MOVE AXIS -> Z-Axis, and move the build plate bed DOWN between 175mm and 200mm, so it is far away from the print head.
5. Turn the printer back OFF!
6. Behind the rear Z-Axis worm-screw rod, there is a hole in the back panel covered by a black rubber plug.   Remove this plug and set it aside.
7. Route the LONG cable for the accelerometer from the inside of the build chamber, through that back panel hole, and connect it to the visible
   white socket on the WiFi module.   Connect the other end of that long cable to the accelerometer, and assemble the two plastic parts as shown on the help article that is linked above.
8. Remove the silicone sock from the print head heater block, being careful not to damage the small wiring above it.  Set it aside.
9. Gently push the open end of the accelerometer assembly onto the nozzle and heater block assembly.  The accelerometer 3D print is intentionally stiff for this rectangular opening, and may require some gentle but firm force.   Be very careful about twisting off axis, to avoid bending your heat block or heat break.   If you have the right tool or finger nails, you may be able to gently spread the two long sides of the accelerometer plastic to get it started sliding onto the print head heater block.   Push it up until it is firmly seated.
   NOTE:   The accelerometer cable will indeed try to rest directly against the Z-Axis vertical worm-screw rod, and when you later start the calibration, the Z-Axis may move (home), but this will not damage the cable.  It may get a small amount of grease on the cable that can be wiped clean when all done.
10. Turn the printer back ON.
11. Use the SETTINGS menu on the LCD panel to select INPUT SHAPER and then Calibration.   Follow the on screen instructions.   NOTE:  Do not panic, but the process will HOME the system and bring the build plate into contact with the bottom of the accelerometer.   This is why the accelerometer bottom is flat and plastic.  Nothing will be damaged.  When the calibration is completed, the values are automatically stored.  There is nothing else you need to do.
12. Use the SETTINGS menu on the LCD panel to select PHASE STEPPING -> Calibration.      Follow the on screen instructions, and the values will be saved automatically when the process completes.
13. When done, exit the settings menu.
14. Ensure that the build plate and bed are moved down and away from the print head,  and Turn OFF the printer.
15. Carefully and gently remove the accelerometer from the Print Head assembly.  It may take some downward force.   If you can fit a finger nail around the top edge of the accelerometer black plastic (without harming the heat block small wiring), and spread the plastic outward it may help to start sliding the accelerometer loose, downward, and off the print head.
16. Put the accelerometer, with it's long  cable still attached, in a storage bag or box.
17. Re-install the silicone sock on the print head heat block.
18. Re-install the small rubber hole plug onto the back panel behind the rear vertical Z-Axis worm-screw rod.

I hope these instructions can help other new users until Prusa can re-write that above linked help article.
As you can see, there were a fair number of questions and concerns for a first time user that were not provided
in the article.   But the Technical Support Chat option was very good, and the support team is great, so I was able
to figure it all out with very little delay.

Cheers,

Neal

I just finished building the Core One + Kit.  I powered it up and LCD stayed black.  Some LEDs under the LCD are blinking (green and other colors).

I lookes around and people talk about reseating the LCD cables.  So I tried reseating them, first at the LCD with no luck and then at the mainboard still no luck.

I found that I could make it worst thoug.  Getting a flashing red light under the LCD for instance and what seems to be a non bootable system if powered-up with a USB thumb drive.

I think I managed to get a firmware update (the latest from March 2026) as it booted, some lights under the LCD and rebooted.

I`ve never seen anythin on the LCD, only flashing LEDs under it.  Reset button works……. Not sure what to look for now.

Any ideas?

Regards,

Normand

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.

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.