I have a hp officejet pro 8500a and it has started leaving a black line on all copies when the original is fed thru the ADF. What is causing this??

Hi, Change first black color print head cartridge just open cover then replace black print head cartridge.close cover wait for ink cartridge reinitialize if ready print test again.if same print having black line all copy needs to replace also black ink cartridge.

The sheets fed thru the ADF are traveling in front of a thin piece of glass (having the length a little bit bigger than the sheet's width). Try to lift the scanner's lid, locate that piece of glass and try to clean it.