I have a 03 td4 and use this one I can monitor most of the engine sensors as far as reading codes not sure as I don't have any issues at the moment and I believe if it isn't broke don't fix it so haven't forced any codes up is unplugging something.
Android Torque Car ELM/327 OBD2 OBDII Bluetooth Diagnostic Interface CAN BUS
I brought it off of fleabag