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:

The ECM detects that the HO2S switching time exceeds a predetermined time.

DTC P0140 is a Type B DTC.

DTC P0140 is a Type B DTC.

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. If any other HO2S DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle to diagnosis the applicable DTC
2. Operate the vehicle within the parameters specified in Conditions for Running the DTC. DTC P0140 should not set.
3. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

1. Verify that the following conditions do not exist:

•	An exhaust leak near the HO2S--Refer to Symptoms - Engine Exhaust .

•	The HO2S is not securely installed.

•	Corrosion on the HO2S terminals

•	Poor terminal tension at the HO2S and at the ECM

•	HO2S wiring damage

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

•	Incorrect RTV sealant

•	Engine oil consumption

•	Engine coolant consumption--Refer to Loss of Coolant .

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

2. Ignition OFF, disconnect the harness connector of the HO2S 2.

Important: The test lamp is used to load the circuit and may not illuminate.

3. Connect a test lamp between the low reference circuit of the HO2S 2 and the positive battery terminal.
4. Ignition ON, test for less than 0.1 volt between the low reference circuit of the HO2S 2 and the negative battery terminal.

⇒	If greater than 0.1 volt, test the low reference circuit of the HO2S 2 for high resistance. If the circuit/connections test normal, replace the ECM.

5. Install a 3-amp fused jumper wire between the signal circuit and the low reference circuit of the HO2S 2 and verify the HO2S sensor parameter is less than 60 mV.

⇒	If greater than 60 mV, test the signal circuit of the HO2S 2 for high resistance. If the circuit/connections test normal, replace the ECM.

6. If all circuits/connections test normal, replace the HO2S 2.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.