First, what printer do you have.
If the copy from platen looks good without any line, insist to clean very well the CVT glass (the little one).
If the line appear also on print means one of the drum cartridges are affected and must be replaced.
i am getting a thick black line across my copies ONLY when my originals go through the feeder. i have cleaned everything i could...and the thick black lines are still there. HELP