Please check the following points to solve the problem:

• Make sure that foreign material such as a torn piece of paper, sticky notes or
  dust is not inside the machine and around the drum unit and toner cartridge.
   
• The toner cartridge may be damaged. Put in a new toner cartridge.
  Please refer to the instructions, "Replace Toner".
   
• The drum unit may be damaged. Put in a new drum unit.
  Please refer to the instructions, "Replace Drum".