3600dn keeps saying tray 2 is empty when its not

Pull the tray 2 out completely. The finger about tray 2 is stuck. Take yiur finger and push it until it is free and not stiff.

The tray 2 on the 3600 is released form its locked posotion when you push the tray in by a finger that is mounted on the metal base of the printer. If that finger is broken, or what I have found is the customer sets the printer on a cabinet or something else where the middle of the base of the printer is not supported and the middle of the base bows so the the finger does not catch the tray relase any more. I have not been able to find the part # for the finger, but all my problems have been the bowing of the base.

Its the first finger sensor right above the tray 2 that gets stuck. It should move freely.

In my case, on my HP 3800n, on the bottom of the paper tray, about a third of the way in from the front in the center, was a small plastic spring loaded latch, or catch. I manually released this catch, and held it in the "released" position with a piece of tape.
Normally, this catch is triggered by pushing the paper tray into the printer, and is released when it hit the metal bottom of the printer. Apparently, the metal bottom panel of my printer was "bowed" down