Think one element may be the dash indicator. If it comes up with the "P" when you turn the key on then it is communicating. If not it stays a blob. Second indicator is if you use the manual release and it slips into drive/revese it is talking. If a jerk it isn't. Final clue: if once in drive it shifts through the gears, it is communicating. If stays in 1st, it isn't.


Finally a bad shifter will probably set codes, faulty brake switch may not.


ps if I park with the top down, I put my hat over the shifter to keep water (rain, sprinklers...) out.