This error indicates that the belt unit is not properly installed to the printer.
Reinstall the belt unit by the following procedure.
Turn off your printer by pressing the power switch.
Insert your finger into the recess on the right side of the printer and pull the front cover open lever (1) to open the front cover (2) forward.
Press the output tray open button (3) and open the output tray (4).
The fuser unit is extremely hot. Do not touch it.
Take all the four image drums out of a printer, and then place them on a flat surface.
Cover the image drums you took out with paper so that the image drums do not expose to the light.
Turn the blue locks (5) on each side of the belt unit in direction of the arrow to unlock.
Remove the belt unit by holding the blue lever (6).
Hold the handle of the belt unit with one hand and support lower part with other hand. Place the belt unit into the printer.
Turn the blue locks (5) on each side of the belt unit in direction of the arrow.
Return all the four image drums to the printer.
Close the output tray (4) by pushing the both sides of the cover firmly.
Close the front cover (2).
The front cover cannot be closed if the output tray is not closed securely.