We need to know the direction of the lines. If you could post a sample that might help. If the lines go from left to right across the paper then the issue could also be the high voltage ps. This was an issue with the old 4000 series. Also what type toner are they using. HP original or aftermarket? Keep in mind customers also do not always tell the truth. I can not tell you how many customers insist over the phone that they changed the toner only to admit when on site and the toner is the issue that they could have sworn they changed the toner.
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