We tried replacing the black toner cartridge -- I was SO sure that couldn't be the problem, because it was almost new! -- and that did it. Somehow the @#$%^ cartridge was the issue.
No we haven't. The solution given to us was already tried (which we wrote into the original post). Unless the new component (Transfer Unit) or the new black cartridges were all defective.
Have you been able to find a solution?
Thank you for any help that you can offer!
Did you ever figure out a solution to this problem? I just started having the exact same issues -- faint screen of black toner under everything, "Check cartridge" message re: the black cartridge. Hoping you figured something out and wondering what you did if so.
I searched the Knowledge-base and found this solution for toner on background of copies.