is Brother's official position on the Drum Error indication. I think you've been through it already, though it doesn't harm to review it (once.)
If using the cleaning slider for the corona wire does not clear the error, the next thing to do is to examine inside the printer, as to whether toner has been leaking from any or all of the toner cartridges. If so, loose toner will be visible (or wipable, off) the inside surfaces of the printer near the toner cartridges or under them.
If such loose toner is found (even a little), then try cleaning all metal contacts, with a clean dry high-quality (and hopefully light-colored) lint-free rag cloth (careful to not wipe or touch or spray the shiny blue-greenish light/touch/moisture/etc sensitive drum.) If one of them is dirty, it might cause the drum or components of it, to register as defective.
Such contacts that would affect the drum, would be either those on the drum unit, or the ones corresponding to them, on the inside of the printer where the drum unit fits in.
And after trying that, if the error still isn't cleared, you could try replacing the drum unit.
(Try a hard reset first, exactly as follows, before ordering or consuming a new drum unit, I would advise:
Turn the printer OFF, when it is on, by suddenly and sharply unplugging it from the power source.
Also unplug it from the computer!
Leave it that way for 10 minutes.
Plug it back in.
Turn it on if it didn't turn itself on. Wait for it to initialize.
Try printing again. )
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