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.

The HO2S utilizes the following circuits:

P0133

P0139

This DTC runs with DTC P0420.

P0133

The ECM detects that the HO2S 1 rich-to-lean or lean-to-rich counts are less than 60 counts.

P0139

The ECM detects that the HO2S 2 response time is greater than 25 seconds after DTC P0420 runs.

DTC P0133 and P0139 are Type B DTCs.

P0133

The ECM commands open loop fuel control.

DTC  P0133 and P0139 are Type B DTCs.

Schematic Reference

Engine Controls Schematics

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Important: In extreme cold weather conditions ice may form in the vacuum source to the MAP sensor and may set a DTC. Inspect the MAP sensor and the vacuum source for ice, moisture, or other restrictions.

1. Ignition ON, observe the scan tool MAP sensor pressure parameter. Start the engine, verify the scan tool MAP sensor pressure parameter changes.

⇒	If the scan tool MAP sensor pressure parameter does not change, refer to DTC P0107 or P0108 for further diagnosis.

2. Observe the DTC information with a scan tool. Verify that other DTCs are not set.

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

3. Observe the DTC information with a scan tool. DTCs P0133 or P0139 should not set.
4. 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 none of the following conditions exist:

•	Lean or rich fuel injectors--Refer to Fuel Injector Balance Test with Special Tool.

•	Water intrusion in the HO2S harness connector

•	HO2S wiring harness damage

•	Incorrect RTV sealant

•	Low or high fuel system pressure--Refer to Fuel System Diagnosis.

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

•	Fuel saturation of the EVAP canister

•	Exhaust leaks near the HO2S

•	Engine vacuum leaks

•	Engine oil consumption--Refer to Oil Consumption Diagnosis

•	Engine coolant consumption--Refer to Loss of Coolant

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

2. If all conditions test normal, test or replace the appropriate HO2S.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.