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Do you photocopy with the feeder, but scan from the glass sheet (one page at a time) ?
Or, do you photocopy one sheet at a time, but scan through the feeder?
If the latter of those two, there is a treatment for the problem:
Open the top of the printer, so that the glass sheet is showing. At the left, you should see a narrower strip of glass.
Take a fresh paper towel, and moisten it with warm water a little. Rub the narrower strip of glass to the left, firmly, many times over, with the moistened paper towel. Some black fleck or flakes should come off onto the paper towel. Then wipe clean and dry with dry paper towel, before the glass strip on the left dries.
Then try photocopying again.
If the line is blurry-edged, that is not easy for you to fix... whether it is absence-of-ink or a dark stripe. That sort is usually owing to a severe hardware defect in the scanner bar (that moves under the glass sheet, when you make a photocopy using the glass sheet.) That would require repairs to the printer.
A way of telling between the two types of defect or stripe, on photocopies: as well as making photocopies through the top feeder, also make one test photocopy of a single sheet original document placed face-down upon the glass sheet. If the stripe is also present in such a photocopy, then you have a scanner bar defect requiring repair, in all likelihood.
That's exactly it. If you scan with the feeder but photocopy one sheet at a time on the glass sheet, then this will cure your problem.
Try the cleaning procedure. Then test the result.
And let me know, please. Thanks.
If the thin line on photocopy made with the feeder was fainter, clean that glass strip again - with a little water on the paper towel this time, and really rubbing the glass clean. (Water, just a little, can be needed to clean off a fleck of hardened ink, and considerable rubbing too.)