a ACDelco or Bosch unit is recommended. they differ from front to back in part numbers. go to a local parts store is is a better choice to ensure the proper match. Just replace the one but since you said mileage was suffering before the light came on please read the following for info on an underlying cause. This is a rich mixture detection for the o2 after the converter so you may question as why the forward one did not report it. Most likely though it might just be a fluke sensor.
Circuit Description
The Heated Oxygen Sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The Control Module (PCM/VCM) supplies the HO2S with signal High and Low circuits. Ignition voltage and ground is supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.
The HO2S contains a heater that is necessary in order to quickly warm the sensor to operating temperature and maintain the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. Once the HO2S has reached operating temperature, the Control Module monitors the HO2S bias, or reference, voltage as well as the HO2S signal voltage for closed loop fuel control. During normal Closed Loop fuel control operation, the Control Module will add fuel (enriches the mixture) when the HO2S detects a lean exhaust content and subtract fuel (leans-out the mixture) when the HO2S detects a rich exhaust condition.
Certain vehicle models utilize an oxygen sensor after the catalyst converter in order to monitor catalyst efficiency.
This DTC determines if the HO2S is functioning properly by checking for an adequate number of HO2S voltage transitions above and below the bias range of 300-600 mV. This DTC sets when the VCM fails to detect a minimum number of voltage transitions above and below the bias range during the test period. Possible causes of this DTC are listed below.
An open or a short to voltage on either the HO2S signal or HO2S low circuits.
A malfunctioning HO2S.
A problem in the HO2S heater or its circuit.
A poor HO2S ground.
This DTC is designed to detect an HO2S voltage that remains at a high (rich) voltage for more than 110 seconds during a 130 second test period. This DTC is set under the following conditions:
There is an HO2S circuit fault that results in a false rich exhaust condition.
The HO2S is correctly detecting the rich air/fuel ratio resulting from either a vacuum leak or a fuel control system fault.
Conditions for Running the DTC
No active TP sensor DTCs
No active EVAP system DTCs
No active IAT sensor DTCs
No active MAP sensor DTCs
No active ECT sensor DTCs
No active MAF sensor DTCs
No active misfire DTCs
No intrusive tests (i.e., EGR or Catalyst) in progress
No device control active
The system voltage is between 11.7-18 volts
Rich Test Enable
System in closed loop
The air/fuel ratio is between 14.5-14.8
The throttle position is between 0-50 percent
The above condition is met for 5 seconds
Decel Fuel Cut-off (DFCO) Rich Test Enable
DFCO mode active
System in closed loop
The elapsed time since test enabled is more than 2 seconds
Conditions for Setting the DTC
Rich Test
The HO2S sensor voltage is more than 994 mV for more than 110 seconds
DFCO Rich Test
The HO2S sensor voltage is more than 469 mV for more than 40 seconds
Action Taken When the DTC Sets
The Control Module illuminates the Malfunction Indicator Lamp (MIL) if a failure is detected during 2 consecutive key cycles.
The Control Module will set the DTC and records the operating conditions at the time the diagnostic fails. The Control Module stores the failure information in the scan tools Freeze Frame and/or the Failure Records.
Conditions for Clearing the MIL/DTC
The Control Module turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.
A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the Engine Coolant Temperature is more than 70°C (158°F) during the same ignition cycle).
Use the scan tool Clear Information function.
Diagnostic Aids
Important: Never solder the HO2S wires.
Check the following items:
The fuel pressure: If the pressure is too high, the system will run rich. The VCM can compensate for some increase; however, if the pressure gets too high, the DTC P0138 may set. Refer to Fuel System Diagnosis. See: Fuel Delivery and Air Induction\Testing and Inspection\Component Tests and General Diagnostics\Fuel System Diagnosis
A rich injector. Perform an Injector Balance test. Refer to Fuel Injector Balance Test with Tech 2. See: Component Tests and General Diagnostics\Fuel Injector Balance Test
A leaking injector. Refer to Fuel System Diagnosis.
Fuel contaminated oil
EVAP canister purge: Check for a fuel saturation. If full of fuel, check the canister control and hoses. Refer to EVAP Control System Diagnosis. See: Component Tests and General Diagnostics\Evaporative Emission (EVAP) Control System Diagnosis
Leaking fuel pressure regulator diaphragm by checking the vacuum line to the regulator for fuel.
The TP sensor: An intermittent TP sensor output causes the system to run rich due to a false indication of the throttle moving.
False rich indication due to silicon contamination of the heated oxygen sensor. This DTC, accompanied by a lean driveability condition and a powdery white deposit on the sensor, may indicate a false rich indication.
An intermittent may be caused by any of the following conditions:
A poor connection
Rubbed through wire insulation
A broken wire inside the insulation
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. See: Symptom Related Diagnostic Procedures
