Brother HL-4200CN - Manual (Page 114)

5 - 25 TROUBLESHOOTING
4
Gently wipe the density sensor window with a clean
dry cloth or cotton swab.
Do not allow anything hard to come in contact with
the density sensor window. Do not use excessive
force to clean the window.
5
Replace the transfer roll cartridge. Grip the orange
U-shaped tabs as shown.
6
Insert the pins at both ends of the transfer roll
cartridge into the bearings inside the printer.
7
To lock the transfer roll cartridge into the printer,
push down on the tabs until you hear a click.