Here is some info from the repair manual about this code. It is a bit technical, but the problem points to an issue with the oxygen sensor circuit. The sensor is supposed to operate within a specific voltage range, and the control module measured a voltage above that range, meaning there is a fault in the wiring, sensor or the control module.
Trouble Code: P0138 (3.5L V6 VIN G Auto) O2 (B1 S2) Sensor Voltage High Condition: Trouble Code Conditions: Engine runtime for 119 seconds; O2 Sensor heater temperature is more than 662ºF (350ºC); Battery voltage more than 10.99v. O2 Sensor voltage is above 3.7v for 60 seconds (300M, Magnum, Pacifica), or 3.99v for 60 seconds (PT Cruiser, Sebring, Stratus)
Possible Causes: * O2 Sensor signal circuit or return circuit shorted to voltage * O2 Sensor has failed * O2 Sensor signal circuit or return circuit open * PCM has failed