Brother MFC-9420CN Service Manual page 257

I-6 White Line (1)
Phenomenon
Vertical white line appears in the specific color area when test-printed in the four color mode (Stripe
Mode), as shown in print sample [6] of Fig.9-1.
Main Causes
(1) Foreign particles adhere to developer roller of specific color in question.
(2) Developer roller's surface is damaged.
Countermeasures
(1) Implement the test print.
(2) Confirm the toner cartridge PU of specific color in question that has caused the white line.
(3) Remove the foreign particles adhering to the developer roller.
Belt cartridge PU
OPC belt
Toner cartridge PU
(K, Y, M, C)
Developer roller
Fig. 9-7
9 - 3 0
Toner cartridge PU
Confidential