Please check the following points:

• One of the toner cartridges may be damaged. Replace the toner cartridge that matches the color of the line.
  Please refer to the instructions, "How do I replace the toner cartridge?".
  
  
• The drum unit may be damaged. Put in a new drum unit.
  Please refer to the instructions, "How do I replace the drum unit?".
  
  
• Clean all four corona wires (one for each color) of the drum unit by sliding the green tab.
  Please refer to the instructions, "How do I clean the corona wires?".
  
  
• Clean the laser scanner windows with a soft cloth.
  Please refer to the instructions, "How do I clean the laser scanner windows?".