Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If the P-9 and P-21 errors appear on the computer's screen, then you have a problem with the server certificate, perhaps, according to what listings I have on K-M c360 errors. Certificate errors can sometimes happen because one or another machine is set to the wrong date (e.g. wrong year but right day and month can do it) - or if all machines are set alike, but the source for their setting is not accurate.
If that is an accurate interpretation of the error codes you cite, if those errors are on the computer not on the printer's screen itself (and they should be), then you must treat the toner problems altogether separately.
What replacements have been made in the printer lately? and when was the last drum replacement for black ? print going faint and then away altogether, unless the drum is conspicuously new, can often be from steady usage of the black drum by text printing.
So if the black drum is definitely not new, and if text printing has been a fairly predominant use of this printer, I would suspect the black drum has gone bad. (But until confirmed, if you try replacing the black drum, save the 'old' black drum in the new one's packaging, in case it is still good and can be re-used.)
I researched the error codes, and found they are listed as meaning
Black Drum or Developer Unit Failure
Color Registration Test Pattern Failure.
Which I understand to mean that print is not being produced in black, basically, and that it is *not* a toner failure or toner supply failure.
The old toner tube may have looked empty because all the toner had gone down the tube but has not quite been exhausted in the workings of the print system that actually use toner.
When drum or developer are listed as problems, then I would say you need to try replacing one or the other.
To find out whether the motor is turning the developer? observe whether there's a distinct ridge of toner deposted upon the developer film in one location along the film's loop - without damaging the developer film by exposure to light or touching it! which would definitely require that the developer be replaced. You wouldn't have to turn the developer film to see it - it should be right there, if toner is being supplied to the developer but none is being used nor scattered along the developer by the developer being turned.
When you look at the inside of the printer, is there any loose toner or spilled toner in the works anywhere?
That might cause apparent or real failure of the developer.
Also exposing the developer to light a lot, or (forbid!) scrubbing it or wiping it, might cause failure.
If you didn't keep the developer in the direct bright light for over 1 (60-second) minute, and if you didn't touch the developer film, then it isn't damaged I doubt.
But if the problem is intermittent, it might be quite expensive: the power supply or high voltage power supply, perhaps. An intermittent problem is likely one that is owing to poor contacts in the printer.
Have you tried the printer since you took the developer out to examine it? does it still show error messages? can it do a test print?
Is the developer unit for the black only?
Or is it for all colors?
If for all colors, then since other colors print, it cannot be the developer. (We should have thought of that first... but I apologize for myself.)
If the developer is common to all colors, but the problem is only in the black color, then the problem must be the drum (mounting, for instance, or dirt on contacts, etc.) or some other black-section-only problem.
Try removing, briefly examining the area where it fits for loose toner, debris, anything broken... then replace, properly, the black color drum.
And see if that helps, or brings back the black printing, at all.
OK. Let's see how it goes, then, being well-prepared. Let me know how it goes, and get back to me at your convenience. Thanks.
If it's an optical sensor, cleaning it means finding it and wiping it with a clean dry rag cloth.
If it's some other kind of sensor, 'cleaning' would only mean picking debris or paper scraps off of it or out of it.
IDC means Image Density Control. There are usually two of them, near the transfer belt.
Vaccuuming dust off them, after removing the drums, developer, and transfer belt, is usual.
I have as yet been unable to locate an illustration of the position of the IDC Front sensor.
But the density sensors are still at issue, whether the fault code is P-5 or P-9.