My HP LaserJet 5P printer has a dark line down the

Most likely is the fuser but you need to know at what point the line is being placed on the paper which will require you to do some testing. You need to do a test page and then shut the power off to the printer when the paper is appoximately half way thru. Then open the lid and remove the cartridge. Look at where the toner is applied to the paper under the cartridge up until before entering the fuser (DO NOT TOUCH THE FUSER VERY HOT WILL CAUSE SERIOUS BURN), do you see the line? Or does the line start after exiting the fuser? If the later then you need a fuser.