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...
Best case scenario is bad hard drive. Try disconnecting the hard drive and boot up the machine. If it comes up ready, then probably bad hard drive.
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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...
Not sure on that model. Don’t have finishers in the field. Thought it might be similar to the 1730 which had a drive assembly inside the rear cover....