The corona wire of the drum unit may be dirty. Clean all four corona wires (one for each colour) inside the drum unit by sliding the green tab. Click here to see how to clean the corona wires.
After completing the cleaning, make sure the green tab of the corona wire is at the home positions (1).
The arrow on the tab must be aligned with the arrow on the drum unit.
The toner cartridge may be damaged. Install a new toner cartridge as follow:
The drum unit may be damaged. Identify the colour causing the problem by using the chart printed in Step 4, and install a new drum unit of the colour. Click here to see how to replace the drum unit.