There are two main types of photocopier machines; mono component and dual component. Mono-component copiers need no developer while dual component use a magnetic powder called developer to complete the copy process. Toner is mixed up with the developer and later on transferred to the drum. Almost all dual component photocopiers use a reference level to control the amount of toner in the developing unit. Failure to maintain the required amount of toner in developer may result in poor copy quality or ghosting. Konica Minolta photocopiers display error code C2557 whenever a problem is detected in developer: toner ratio. Most likely this error will be displayed if toner bottle is empty. Konica Minolta recommends changing developer and imaging unit simultaneously in Bizhub 250, Bizhub 350 etc; if you replace only developer and not the drum it may display error C2557. Solution is simple, open right side cover of the machine. Door open message will display. Take drum unit out of the machine. Now enter to the service mode; press Utility/counter key, touch “Check details” on the LCD display, press “Stop-0-0-Stop-0-1”. Enter password 00000000 (eight Zeros), touch “End”. Select “Counter” and clear the life count of developer and PC (select than press C button). Now return the drum units to the machine, close right side door, touch Functions than F8 from tech rep main menu and press Start key. Start key will begin to blink and will turn into green when process is complete. Restart the machine. Alternately you can change the toner reference level manually to approx 180. This may increase toner amount in developer, so be careful while trying this method.