Have same problem recently on C1085.
I dont have my log book to look now but i think it was this error C-4661 and sometimes C-2220.
When machine rarely managed to turn on without errors prints have high color registration mismatch,that guide us to think about this sensors fault readings.
Before, i tried to replace color registration sensors,no success,check transfer belt...
Solution was to clean this sensors,and yes you have to access from back.
You can get to them without too much disassembly but can not clean proper without removing them out completely.
My colleague done that and i take a look from time to time and i see its pretty much hard to remove them (must remove board,fly wheels,some cable harness,...)
Can not guarantee with my life that is your problem too but i am pretty sure.
After all you done already worth trying this at least and if it solve your problem you can thank me
Ps:i have more than once situations when solution was not written in service manual.
Bookmarks