What causes the 'open tray' tray error?

Cause #1 - Dusty laser, sometimes the laser can get quite dusty so I would recommend trying a lens cleaner

Cause #2 - If the problem is not a dusty laser, then laser module may not be at correct starting position. By this I mean the laser position should right under your game disc instead of it being in a position that is outside the disc area where there is no data to read. The only way to fix this is by modding your drive to stop it getting stuck.

Cause #3 - If the starting position is ok and still showing open tray then the laser may need to be pot tweaked to make it start reading again

If all these fail then a laser or drive replacement will be needed