DIAGNOSIS Toner is not being fully fused to the paper or is transferring from a dirty component. This is typically a fuser or drum unit issue.
FIX Determine the smudge pattern. If the smudge repeats at regular intervals (e.g., every 3.5 inches), it is a drum unit problem. The photosensitive drum has a scratch or is coated with old toner. Remove the drum unit. Clean the green drum roller thoroughly with a lint-free cloth. Rotate it to clean the entire surface. Reinstall. If the smudge is irregular, it is a fuser problem. The fuser roller may have a toner buildup. This requires fuser cleaning or replacement.
IF THAT DIDN'T WORK The problem may be the toner cartridge itself. Faulty toner or a damaged cartridge can cause scattered toner to deposit before the fuser. Try a different, known-good toner cartridge as a test.
HEADS UP Only clean the fuser unit when the printer is completely cool. Use a specific fuser cleaning web or a soft cloth lightly dampened with isopropyl alcohol. Abrasive cleaners will destroy the roller.