The 1-2 sensor is indeed the after cat sensor on bank 1. Bank 1 is the one which has cylinder #1, which is the left bank as seen from the drivers seat for Chrysler products.
The P0138 code itself could mean a short in the wiring to the sensor, sending a voltage of more then 0.999 V for a prolonged time to the PCM. Check the O2 sensor wiring first for any shaving or melted wire isolation due to contact with hot exhaust parts. If the wiring seems to be ok, then most probably the O2 sensor is bad.