I have a Dell B3465dnf and it worked fine until I replaced the toner cartridge recently.  The old toner was low so I replaced it with a new toner.  It printed a very dirty black background.  There were a lot of black particles on the image unit and they came back immediately after cleaning.  I had to switch back to the old toner and the problem went away.  Now the old toner is completely empty.  I have tried two new toner and the same problem appeared.  Someone has suggested to replace the image unit.  But I think the image unite is not the problem.  The image unit is low but it should not cause this problem otherwise the old toner should not have worked.  

Thank you very much to help me to figure this issue!

Yan