Canon LBP251dw User Manual page 342

Printing speed becomes slower when you select stronger improvement.
Special Printing Mode D
Streaks may appear on printouts depending on the paper type or the environmental conditions. Setting this
item to <On> may solve the problem. Also, setting this item to <On> will reduce the operating noise during
printing.
Off
On
<Adjustment/Maintenance>
D>
If you set this item to <On>, the printing speed becomes slower.
Clean Fixing Assembly
Clean the fixing unit after a toner cartridge is replaced or if black streaks appear on printouts.
Setting Menu List
<On>
<Special Processing>
336
<Special Printing Mode
Fixing Unit(P. 394)