|Then why would you suspect a factory defect? |
Does the unit print documents without the line?
Does the line appear if you print a scanned document?
Scanners are not airtight and foreign matter can get inside. In addition to that, there is considerable heat generated when scanning. I mention this not to imply this is your problem but to point out possibilities.
Is the line exactly in the center of the scan?
Is the line well defined?