grayscale only? Try to print something in gray only...and if you get lines, then it is your black ink.

Sometimes you get lines when the cartridge is not cleaned properly.

Our black did that but still had ink in it. So I carefully cleaned the holes with alcohol (it says not to do it--but it worked)

Then we ran a test sheet and the lines were gone. We don't print during the summer when we are
not here and we unplug the electrical lines (so the unit goes off and cannot maintain itself).
Our new HP does "stuff" by itself...sort of creepy, but now we think it is cleaning the cartridge.