This is the old chipless generation, so it's a much easier fix at least :-). Keep this unit until it needs major service and try and find a low meter printer that lacks the chips if you can, the chip printers can be a royal PITA with recognition issues. Brother doesn't explicitly pull an HP or Xerox*, but they are a little sensitive.
*Only a problem on pre Lexmarkified engines and floor printers with metered supplies. They just nag you briefly if it's flagged to run Sold supplies.
First thing I would check is the toner flag and make sure the toner isn't off balance - make sure it is set to "full" - use your yellow or magenta as a baseline, as those are full or barely dented. You want the gear to be in the full position as reusing a toner will trigger this unless you reset it on the gear/optical toner printers like the L6850. You can reset it if needed by removing the cover, pulling the spring back and forcing it back into the full position if the gear isn't at 100% capacity. I suspect the chip printers do not have this problem but until Brother lets us print on "empty" toner like HP, Lexmark and Canon I don't buy these - but on the other hand, I would take one in if someone gave me a gently used one for the sake of giving it a home where the repairs are viable.
If the gear is in the full position, check the ground points on the drum unit as well as the toner - if those are dirty, it causes a similar problem. Wipe it down with a microfiber cloth you can sacrifice (that is to say, one you can throw in the garage and never use on things like prescription glasses). If the toner didn't come with one, buy one on eBay or Amazon and add it. That will fix the problem if the toner didn't come with one or it is not happy with the one it has installed.
If neither of those work, it's usually a drum issue. It's a combined CMYK part so it's on the expensive end of the spectrum. If you can take it apart a bit or clean it I would go that route before spending that kind of money on a new drum unit.
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@porpoise1983249 "Could this be a Brother ploy to get you to buy their vastly over-priced cartridges?" that is exactly what I was thinking as well. Let's see if our printer guru @nick has a solution for you.
crwdns2934271:0crwdnd2934271:0 oldturkey03 crwdne2934271:0
@oldturkey03 This one has the chipless toner so there’s ways to try and recover it.
crwdns2934271:0crwdnd2934271:0 Nick crwdne2934271:0