If you have print quality problems, clean the corona wire.
Press the front cover release button.
Keep the button pressed and pull the cover toward yourself.
With the front cover in the downward position, the drum & toner unit will be visible.
Grab the black-and-yellow handle on the unit. Lift unit up and pull it out of the printer.
Clean the corona wire inside the drum unit by gently sliding the blue tab all the way to left and then back to the far right. Repeat several times.