A Vertical Black Streak
The wiper blade is worn — it isn't cleaning toner off the drum. The wiper blade needs replacing, sometimes along with the drum.
A Vertical White Streak
The toner has run out or clumped in one spot. Remove the cartridge and shake it horizontally. If that doesn't help, the laser window is dirty — wipe it with a dry cloth.
Repeating Spots at a Consistent Interval
Measure the distance between spots: 75-95 mm indicates the photo drum, about 50 mm the magnetic roller, and 110-130 mm the fuser roller.
A Gray Background Across the Whole Sheet
Low-quality toner or a worn drum. This often appears after refilling with cheap powder.
Text Rubs Off with a Finger
The fuser isn't heating to the required temperature, or the paper is too heavy for the selected mode. Check the media type setting in the driver before replacing any component.
A Smeared Printout Along the Edge
The paper path is dirty, or the waste toner container is full.