Well, that's a pretty clear pattern (and by the way, I must praise you for making the effort of scanning and sending me a typical print-defect page, that helped my process of diagnosis enormously.)
What you have is an odd-shaped non-sharp-line type defect down the page, repeating a pattern at sufficient intervals that it is present 3 times or more on every printed page.
This sort of repetitive defect is an actual physical problem on one of the roller-like rotating elements in the print engine.
I think it most likely to be the photosensitive drum or roller, which is very easily damaged by a whole list of things: dampness/moisture, exposure to light, abrasion/scratching, being touched by a bare human hand (which deposits an oil that spoils the delicate material of the surface quite quickly), etc... maybe hard radiation too, but I don't know about that.
It looks like a damaged photosensitive drum defect to me. The lighter stripe up the page might be owing to trying to wipe the ...whatever it was that created the repeating pattern... away physically (which incidentally is always a mistake. Touch damages these things, period, one cannot wipe them off or away.)
In this model, I suspect that the 'drum' is part of the toner cartridge (in many modern machines it is.) If not, then replace the 'developer' or the 'drum kit', if there is such a thing for your model of machine (and there doesn't seem to be, so I suspect the ultimate correct diagnosis and recommendation is:)
In short, replace the toner cartridge. It ought to improve at once. (And take care when handling it, of course.)
This picture shows clearly that the sensitive 'drum' is indeed a part of the toner cartridge for this model:
- that's the greenish shiny roller element. Do not touch it, sneeze on it, wipe it, expose it to direct fairly bright light for over a continuous minute per half hour [this causes a sort of 'overexposure' or permanent chemical alteration.]
PS: if you know how to inspect your fuser rollers in your printer, do so also. It probably isn't damage to the fuser roller at all, but it's just as well to check. Such damage would be very visible (and frankly this doesn't look like that at all, it would be much more overt and sharp-edged if it were fuser roller damage, I would think.)
Good wishes for your future use of this printer.
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