I have a 4 in for cleaning so I verified the following as a quick test. The rear door sensor is activated by a plastic fin on the rear door (drop the door open and it is on the far right about halfway up. The fin is taller than the paper guides and further up the door. If the fin were broken off, the sensor would register a paper jam.
The fin activates a lever that can be manually moved when the door is open. Look on the right side at the height of the fin (not far under the rollers) andthere is a black plastic piece that can move a little back and forth. That isn't it - but the end of the lever is the piece of black plastic straight inside from that. It can be pushed in with something thin like a pencil (a screwdriver, if you must). If the sensor is operating AND nothing else is wrong, pushing in the lever while the printer is on will clear the paper jam warning and bring the printer Ready.
While you have the door open, you can push down on the fuser assembly door and look inside for paper caught there. You can pull out the entire fuser assembly for a look at the interior by taking out the two hex-head screws on the metal plate near the bottom of the door and pulling straight out (power off and cool the fuser first,please).
