Super late response, but I ran into this thread from a casual google search, figured id chime in..
P0820 refers to the cream little square box with a connector that looks like it comes out but is not supposed to, sporting yellow, red green, black wires.. its located at the top and rear of the transmission ( at the gear box)
that box is the xy position sensor.. there is some info about it starting in TM-48 of the FSM
IF you got the DTC, most likely
A: the harness is disconnected from the short cable going to the box
B: the short cable was torn out of the box (bad)
C: Something happened to foul the box (bad)
D: The box is otherwise fouled or wire harness fouled elsewhere carrying this signal
I personally ripped out the short cable to the XY box. the result, is loss os S-Mode, loss of shift indication on the dash below the tach, no reverse light... The car continues to operate as normal otherwise.
A new XY Position sensor costs about as much as a used rev match transmission. You would never buy this used, since you cant guarantee it wasn't knocked around or placed near something magnetic during removal ( only if you bought it from a used tranny and swapped it yourself.)
hope this helps