Does it happen on prints and copies? Probably a drum.
But if only on copies I would then assume it is a straight line and only happens when using the document feeder, and if that is the case I would further assume that it is a solid line that goes oerfectly straight across the page in the direction the paper is fed. And again, if that assumption is correct, lift the document feeder and look at the narrow strip of glass, probably on the left side, I believe you will find something on the glass, typically a solid (whiteout). If so just scrape with the side of a bank card/ ID card and wipe with a rag (Don't use windex)
Obviously I have made assumptions and this may not be helpful.
This pic has the piece of glass circled I referenced
Usually when buying a machine you get a toner thats not even half full, when buying new toners you will be able to print a lot more.
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I have a 6015 that I bought in August and it's already printing a black vertical line when I'm copying. I read in the forum that I should clean all the LED windows and nothing seems to work. I can't believe that the toner is already empty I bought it in August as stated earlier and maybe did a total of 100 copies if that. Do I need a service call on that or what would be the next step because if I have to replace the toner already after 100 copies well this machine is no good period