The Malfunction Indicator Light (MIL) on and the Powertrain Control Module (PCM) is setting code P0141.
1. Check the White/Black (WH/BK) wire at Powertrain Control Module (PCM) pin 95 for battery voltage with the ignition key in the run position.
2. If there is battery voltage on the WH/BK wire, disconnect the rear O2 sensor on the passenger side to verify that the voltage goes away. If not, this circuit is shorted to a power feed. Locate the source of the voltage feed and repair as needed.
3. If the WH/BK wire shows battery voltage, and it goes away when the sensor is unplugged, reconnect the sensor and start the engine. Monitor the HO2S12 Heater status on the scanner. When the scanner shows the heater is ON, verify that the voltage reading drops to less than 1.0 volt. If not, the PCM is defective or this circuit is making poor contact to the male terminal of the PCM.
4. If there is not battery voltage on the WH/BK wire, verify there is battery voltage on the Red (RD) wire to the sensor and check the resistance of the sensor between the two White wires of the sensor.
5. If sensor resistance is OK, and battery voltage is OK on the RD wire but not the WH/BK wire, service the WH/BK wire for an open or short to ground between the PCM and the sensor.
Oxygen (O2) sensor #12 is the sensor downstream of the converter on the passenger side. Oxygen (O2) sensor heater element resistance should indicate between 3-30 ohms, checking between the two White wires of the sensor.