The issue is most likely a broken Drum unit so you just have to replace the R3 Drum Unit Magenta
I just wanted to post an example of this before I try to find a local Xerox service company that will actually respond to me.
You can see that the Magenta toner only seems to get to maybe a third of the page. It doesn't fade out slowly across the page, nor does it seem to fade out from page to page as I might expect if the supply of toner was blocked in some way. It seems as though whatever makes Magenta mix in with the other toners is only working for a third of its length. I'm guessing without any great knowledge of how the process works
I see no one has responded to this post yet. So, at this point I would recommend contacting your local Xerox Support Department to speak with a Product Specialist for more assistance.
My ageing 7125, in the middle of a long print run today, stopped using magenta for the bottom two thirds of each page. I've tried shaking the toner cartridge, cleaning the lens etc. etc. so any suggestions on that would be welcome ...
... or ...
... do Xerox have recommended service/repair engineers on the North East London (UK) area? Googling for that throws up thousands of possible candidates but, given the choice, I would rather pick someone recommended by Xerox. I inherited this printer many years ago when the company I work for changed all their machines and service contracts. Clearly I can't really justify a continuous service contract, but a good service guy might be able to keep it going a bit longer. It would be such a shame to discard it for the same of a little TLC from an expert