Try updating the firmware (via SD card) if this works then most likely the controller board is OK. It the device throws the SC672 then you'll need to...
I can't tell you which models, but once they go into an error code the Settings Menu may become inaccessible. And if you replace the printhead while...
I've never seen the official bulletin, but it's no small task. On later production models they installed a new style front cover with holes to access...
If I were to guess, I'd guess that the U021 reset cleared the speed upgrade code, so it won't accept that toner, and it will run at 25ppm. I suspect...