Remove MMU3 from MK4S
I bought an assembled MK4S with MMU3 and I'm having a lot of issues and just need it to work. (It's getting MMU overcharge and filaments have trouble loading until I reset the printer)
Since it was pre assembled, what do I need to do to turn it into a regular MK4S? What do I need to change? Sensors? HF nozzle?
RE: Remove MMU3 from MK4S
Just take it off the top mount and set it aside, remove the tube feeding the extruder, and put the regular filament holder in its place. All you have to do is select the regular (non-mmu3) extruder in the slicer and you are good to go.
RE:
you could simply disconnect the MMU. however the filament sensor would not be ideal.
alternatively revise the Nextruder to use the normal filament sensor
Print (or buy) the parts you need for the re assembly of the normal filament sensor, before you disassemble the nextruder.
then disassemble the nextruder to remove the MMU Style Filament sensor components
I am having difficulty finding the assembly guide for the Mk4 (MMU3 version) Filament sensor
If you look at the MMU3 printable parts list
https://www.printables.com/model/531604-mmu3-printable-parts/files
there is a section for parts used in the Nextruder. 
these need to be replaced by the items in the link below...
re assembly see instructions in the link below
https://help.prusa3d.com/guide/5-nextruder-assembly_434014
I believe the filament sensor spring is
https://www.prusa3d.com/product/spring-3x9-set/
I can't see the tiny ball bearing, Magnet, and prusa ball holder in the spares list.
I suggest contacting the Chat team to ask for these parts.
Don't forget to load the correct firmware, and re run the initial configuration wizard
regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
RE: Remove MMU3 from MK4S
The Filament Sensor Assembly is that little black injection-molded part. I'm in the process of returning my Mk3.9 to stock in order to eventually move my MMU3 to my new Core One and cannot find mine. Chat agent pointed to this store page for me: https://www.prusa3d.com/product/filament-sensor-accessories/
All the other parts can be re-printed from the Mk3.9/4 Printable Parts page.
The agent told me to follow these directions, in reverse, basically to restore the Nextruder to stock 3.9: https://help.prusa3d.com/guide/9a-mk4-s-mk3-9-s-nextruder-mod_683106#683464
RE: Remove MMU3 from MK4S
I released a idler for Nextruder that now fixes filament detection completely, take a look...
https://www.printables.com/model/1269218-nextruder-mmu-multi-material-gamechanger