Brother MFC-9420CN Service Manual page 275

I-24 Uneven Density (Right & Left)
Phenomenon
The optical density is different between the right and left side of the printed image.
Main Causes
(1) The amount of toner in the toner cartridge PU is short.
(2) The retract operation of the toner cartridge PU is not properly done.
(3) The belt of the toner cartridge PU is deformed.
(4) The belt of the transfer unit is deformed.
(5) The transfer roller ASSY PU is not properly installed.
(6) The rear cover ASSY PU is deformed.
(7) The rear cover ASSY PU is not locked sufficiently
(8) The dustproof glass of laser ASSY PU is stained.
Rear cover ASSY PU
Transfer unit PU
Countermeasures
(1) Replace the toner cartridge PU.
(2) Check the toner retract clutch PU.
(3) Replace the belt cartridge PU.
(4) Confirm the proper installation of the transfer roller ASSY PU.
(5) Replace the transfer roller ASSY PU.
(6) Ensure to lock the rear cover ASSY PU.
(7) Replace the rear cover ASSY PU.
(8) Clean the laser ASSY PU.
(9) Replace the laser ASSY PU.
Transfer belt
Fig. 9-24
9 - 4 8
Belt cartridge PU
Toner cartridge PU
(K, Y, M, C)
Laser ASSY PU
Confidential