What's your question? my konica minolta c451e printing machine showed me an error code p21, what could that mean and it is not printing any image but black-green shade

REplace the black image unit and print out each of the colors cyan yellow and magenta any of them bad?

P21 & P8 are errors of yellow drum cartridge, replace yellow cartridge.

what model of Konica Minolta?

check The Transfer Belt or it can be the Black Image Unit . (P9 & P21 normally: black Imaging unit) but P21 can also be refered to Transfer Belt). ..

try to go to your printers and faxes it is located in the start menu.

P6 : cyan image unit problem. P21 come after P6, and it's also too a cyan unit image problem. replace the cyan image unit.

replace the black image unit and clean sensors under the transfer belt

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P21 is a colour registration test pattern failure - check transfer belt for damage and or dirt, clean if dirty if damaged replace unit, or possible print head fault

P 21 Color shift test pattern failure.Relevant electrical parts is the image transfer belt unit.
Wipe clean the surface of the transfer belt with a soft cloth if it is dirty.Change the image transfer belt unit if the transfer belt is damaged.Clean the 2 sensors located upper side of unit.

tis error means abnormal tonder density in the balck image unit take the blacke image unit out and shake it while holding it level and put it back in the machine to reset the error code leave the machine on and open then close the front door