I just had this same code on a 6501, how do your test prints look? If your test prints look good check the color Reg. sensor, cln or replace. Mine ended up being the color Reg Sensor unit. The 2804 refers to black patch error in gamma correction.
Re: Konica Minolta Bizhub Pro C6501 Error Code C2804
i have same problem with C5501,error c-2804.
Week ago it start showing daily c-2804 on startup but simply off/on reslove problem.
Print quality was ok.
Now it can not be turned on anymore, c-2804
I tryied clean sensors first,no affect,then i pull out complete color registration unit,developing unit k and transfer belt and try in other machine and it works ok.
One more thing i see is that on transfer belt after pulling out you can see yellow,magenta and cyan tone line but black is missing.
Anybody can help with possible solutions?