The forum is screwed up where your post won't allow replies.
You need to clean the mirrors in the optical section.
You're in luck, two screws to remove the right-side glass cover.
Then move around the optical carriage to wipe down the mirrors with alcohol.
Sometimes it takes two or three cleanings to eliminate the problem.
This happens because the hot fuser melts plastic and fumes carry up to the optical unit.
You need to clean the mirrors in the optical section.
You're in luck, two screws to remove the right-side glass cover.
Then move around the optical carriage to wipe down the mirrors with alcohol.
Sometimes it takes two or three cleanings to eliminate the problem.
This happens because the hot fuser melts plastic and fumes carry up to the optical unit.