I have a LaserJet 4000 with a repetitive defect measuring in at 2-15/16". From the Repetitive Defect Ruler on PrinterTechs.com, this distance indicates either the Tray Feed Roller, the Separation Roller, or the Fuser's Upper Hot Roller is at fault.

I've fed a sheet into the manual feed and turned off the printer when halfway in and there is no defect on the sheet (pre-fuser), but how do I know that I'm not bypassing a problem that may have been introduced by feeding paper from a tray (by using the manual feed)?

In other words, does my test confirm with certainty that the fuser is the problem?

Thanks in advance,
Dave