HP LaserJet 4200 Print Quality Issue - Vertical banding

The attached file shows vertical banding, or more specifically, loss of toner intensity on the same central area of the page on every page that is printed. The transfer roller is new and I would just like a confirmed diagnosis from someone with more experience than myself as to the most likely cause of this print quality issue. Many thanks.
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I would try replacing the toner cartridge, as that is the most likely culprit. If that does not help, there may be dirt on the laser window. In that case, cleaning the window should fix it.
ALLITAuthor Commented:
Thanks hdondt... toner has been replaced already, I should have mentioned that. I have flicked through the manual and cannot see a reference to the 'laser window' - can you guide me as to where this is please? Many thanks.
Not sure where it is oin your printer, but if you look at the toner cartridge you'll see the green drum (hidden by a flap). The laser window is in the printer next to the drum

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