I have the same problem and my machine is in very good condition. I have tried everything and on the verge of throwing out the machine.
The same problem suddently developed for my 6505. There are NO ERROR CODES. From any source, the pages come out black and now tending to horizontal lines of black, yellow and pink with diagonal lines accross the entire page, front and back. Updated the firmware and cleaned everything, but will not go away. The Startup Page and everything prints the exact same pages as described!
I have an older Phaser 6500 laser that worked fine until I replaced the Magenta toner cartridge today. It printed 6 good pages and then began printing solid black pages. I have turned off printer and unplugged for good measure. Shut down computer and restarted. Used both network and hardwired printer descriptions. Tried different source programs for the document to print. Now I am getting faded almost black pages with no sign of the document. Any ideaswould be appreciated.