Brother 4000CN - Manual (Page 133)

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4.
Gently wipe the density sensor window
with a clean dry cloth or cotton swab.
Look
1
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.