DIAGNOSIS Debris (usually a small piece of paper or toner clump) is on the laser scanner window inside the printer. This blocks the laser beam from exposing part of the drum.
FIX Remove the toner and drum unit. Look for the narrow glass strip (laser scanner window) running left to right deep inside. Gently wipe it clean with a dry, lint-free cloth. Do not use liquids. Reinstall the drum and toner.
IF THAT DIDN'T WORK The drum unit itself may have a scratch or defect on its surface. Try a different, known-good drum unit to confirm. A faulty toner cartridge can sometimes cause this, but it's less likely.
HEADS UP Never touch the green surface of the drum unit with your fingers. Skin oils will cause permanent print defects.