The heated oxygen sensors (HO2S) are used for fuel control and catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the engine is started, the control module operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air-to-fuel ratio. The control module supplies the HO2S with a reference, or bias voltage of about 450 mV. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0-1,000 mV. This voltage will fluctuate above and below the bias voltage. Once sufficient HO2S voltage fluctuation is observed by the control module, Closed Loop is entered. The control module uses the HO2S voltage to determine the air-to-fuel ratio. An HO2S voltage that increases above bias voltage toward 1,000 mV indicates a rich fuel mixture. An HO2S voltage that decreases below bias voltage toward 0 mV indicates a lean fuel mixture.

The heating elements inside each HO2S heat the sensor to bring the sensor up to operating conditions faster. This allows the system to enter Closed Loop earlier and the control module to calculate the air-to-fuel ratio sooner.

Fuel trim bias is used by the ECM to keep the post catalyst HO2S voltage within a range of 580-665 mV. This allows optimal catalyst efficiency under light load conditions, such as at idle or a steady cruise. The ECM constantly monitors how lean or rich the fuel trim bias is commanded, to determine if the fuel trim bias is greater than a calibrated amount.

The HO2S utilizes the following circuits:

The post catalyst fuel trim correction factor is biased rich by more than -8 percent of the HO2S signal value for more than 4 seconds or for a cumulative of 50 seconds.

DTCs P2097 and P2099 are Type B DTCs.

DTCs P2097 and P2099 are Type B DTCs.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

1. Ignition ON, observe the scan tool DTC information. Verify DTCs P0137, P0138, or P0140 are not also set.

⇒	If any of the above DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

Caution: Refer to Road Test Caution in the Preface section.

Important: The rear heated oxygen sensor (HO2S) must be in closed loop for this diagnostic to run. A road load condition is necessary to obtain closed loop.

2. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Verify that none of the following conditions exist:

•	Rich fuel injectors--Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2 .

•	Water intrusion in the HO2S harness connector

•	High fuel system pressure--Refer to Fuel System Diagnosis .

•	Fuel that is contaminated--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

•	Fuel saturation of the evaporative emissions (EVAP) canister

•	Restricted exhaust

•	Contaminated HO2S - Silicon

⇒	If you find any of the above conditions, repair as necessary.

2. Ignition OFF, disconnect the harness connector at the appropriate HO2S 2.
3. Ignition ON, verify the appropriate scan tool HO2S 2 voltage parameter is between 350-550 mV.

⇒	If greater than the specified range, test the signal circuit of the HO2S sensor for a short to voltage. If the circuit/connections test normal, replace the ECM.

4. Connect the harness connector at the HO2S 2.
5. Engine idling, raise the engine speed above 1,200 RPM for 30 seconds. Verify the appropriate scan tool HO2S 2 voltage parameter is less than 1,050 mV.

⇒	If greater than the specified value, replace the HO2S 2.

6. Engine idling, move the appropriate HO2S 1 and HO2S 2 harnesses between the HO2S harness connectors and the ECM. Observe the suspected scan tool HO2S 1 voltage parameter, and the HO2S 2 voltage parameter.

⇒	If a parameter changes abruptly, repair the harness/connections as necessary.

7. If all circuits/connections test normal, replace the applicable HO2S 1.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.