Please check the following points:

• Clean the corona wire inside the drum unit by sliding the green tab.
  To see how to clean the corona wire of the drum unit, click here.
  
  
• Make sure the primary corona wire cleaner is at the home position.
  
  
• The drum unit may be damaged. Put in a new drum unit.
  To see how to replace the drum unit, click here.
  
  
• The toner cartridge may be damaged. Put in a new toner cartridge.
  To see how to replace the toner cartridge, click here.