Brother MFC-9420CN Service Manual page 301

I-21 Back Stain
Phenomenon
Back side of the print paper is stained as shown in
Main Causes
(1) Fusing unit:
1) The fuser roller and the back-up roller is stained.
(Print immediately after the inner jam occurred.)
2) Fusing off-set error occurred. (Fusing temperature is not correct.)
3) The fuser roller and the back-up roller are deteriorated.
(2) The transfer roller is stained.
Countermeasures
(1) Clean off the stain of the fuser roller by carrying out the blank printing for couple of pages.
(2) Replace the fusing unit FU with a new fusing unit FU.
(3) Replace the transfer roller ASSY PU with a new transfer roller ASSY PU.
print sample [21] of Fig.9-1.
Fig. 9-22
9 - 4 5
Fusing Unit FU
Confidential