Xerox 6250dp My printer suddenly decided to stop printing blue. The service pages have purple instead of blue. I have changed the print cartridges to no avail. There are light blue squares on the service pages, but no dark blue. New image unit? Image unit 41%. Fuser 95%. Transfer roller OK. 94006 pages. Thanks Bob

defective imaging unit.

Use only original ink cartriges, the non original has ink normaly is not for you printer, and you wast your money and damage the printer print-head

In most cases cartridges have a special chip that remembers the toner cartridge serial number an amount of pages printed from it. This means, that even though there still might be some toner left in the cartridge, the chip's counter reaches it's predefined limit and informs printer, that the toner has run out. Just like that, suddenly.
It may also be the case, that the chip loses contact with the printer's electrical terminals - try reseating the cartridge in the printer. If that doesn't help, it's time to replace it I'm afraid.
Usually the printer will inform "toner low" for some time before it becomes "toner out" and refuses to print. If you get "toner low", then it's time to purchase a new cartridge to have it ready.

Your yellow cartridge is probably not working as your photos suggest. Remove and reinsert the yellow cartridge. Try printing only yellow color.
Your printer has a separate head for printing; the nozzle for yellow ink might be clogged. Try running a head cleaning routine from your printer software.

Your toner supply route is partially plugged and it can't keep up with the demand. It's not an easy process to clean it but a can of air, toner vac, and an area you don't mind getting dirty are necessary. Pull the imaging unit out and dig around, blowing and vacuuming out toner. Also, pull the black toner cartridge and work through that way as well. If your black toner is low, that might be the problem.

I bet you used a Generic toner (Mediaq Science?)
this will contaminate the developer and cause poor prints. Vacuum, Replace developer, replace toner (Xerox Brand). Good to go.

Make sure it's the right ink cartridge for that model printer.
Check if any of the colors is out
Try running a calibration on the printer to make sure heads are lined up properly.

Link to the manual - Aligning print cartridges is on Page 70.
http://h10032.www1.hp.com/ctg/Manual/c00905552.pdf

Your color cartridge is probably empty as well. you can go to>control panel>printers and faxes> your printer >right click on preferences then click on services. Do all of the maintenance possible on your cartridges and see if that helps. Are your cartridges HP cartridges or generic refills?

Is there a specific code coming up? That can make it easier to pinpoint the exact problem.

Yeah. Most are ok. But the best way to go it OEM with any product.
Do they cost more? Yes, but the reliability certainly makes it work the cost.