There are contacts on both sides of the drum unit. Some of them inform about the presence and model of drum, and others convey the information about the toner cartridge inserted in the drum unit.
Please make sure that the contact fields on the drum unit itself, and corresponding springy wire contacts protruding from the walls of the printer interior are clean, and that the black drum unit is properly seated in it's place.

Hi,


A temporary fix would be just open and close the handle and check that the white plastic is back down.

For a permanent fix replace the toner cartridge. Let us know if you need further clarification.
Thanks for contacting fixya.com