Diagnostic Instructions

DTC Descriptors

DTC P0136: HO2S Circuit Sensor 2

DTC P0137: HO2S Circuit Low Voltage Sensor 2

DTC P0138: HO2S Circuit High Voltage Sensor 2

DTC P0139: HO2S Circuit Slow Response Sensor 2

Diagnostic Fault Information

Circuit	Short to Ground	Open/High Resistance	Short to Voltage	Signal Performance
HO2S 2 Signal	P0136, P0137	P0136, P0137	P0136, P0138	P0137, P0139
Low Reference	--	P0136, P0137	--	P0137

Typical Scan Tool Data

HO2S 2

Circuit	Short to Ground	Open	Short to Voltage
Operating Conditions: Engine operating in Closed Loop with high exhaust system temperatures Parameter Normal Range: Fluctuates above and below 350-500 mV
HO2S 2 Signal	15 mV	55 mV*	1,275 mV
Low Reference	--	55 mV*	--
*Voltage varies slightly with sensor/exhaust system temperature

Circuit Description

The heated oxygen sensor (HO2S) sensor 2 circuit diagnostics monitors the voltage on the signal circuit of the post catalyst oxygen sensor. In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the heated oxygen sensors HO2S 1 and HO2S 2. The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2.

The HO2S utilizes the following circuits:

Conditions for Running the DTC

P0136, P0137, P0138, or P0139

P0137

The target air fuel ratio is RICH.

P0138

The target air fuel ratio is LEAN.

P0139

The engine is operating in fuel cut mode.

Conditions for Setting the DTC

P0136

P0137

The average HO2S 2 voltage is less than 250 mV for greater than 30 seconds.

P0138

The average HO2S 2 voltage is greater than 590 mV for greater than 30 seconds.

P0139

The HO2S 2 voltage is greater than 200 mV for greater than 1 second during engine decceleration, fuel cut mode.

Action Taken when the DTC Sets

DTCs  P0136, P0137, P0138, and P0139 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTCs  P0136, P0137, P0138, and P0139 are Type B DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

Important: To ensure the HO2S 2 is active, operate the engine at 2,000-2,500 RPM for 2 minutes, before performing the following verification.

1. Engine running at normal operating temperature and the exhaust system hot, observe the scan tool HO2S 2 voltage parameter. The reading should fluctuate between 250-650 mV.
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.

Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the HO2S 2.
2. Ignition OFF, test for less than 10 ohms of resistance between the low reference circuit terminal 3 and ground G104.

⇒	If greater than the specified range, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the PCM.

3. Connect the HO2S 2 electrical connector.
4. Ignition ON, observe the scan tool HO2S 2 voltage parameter is less than 1050 mV.

⇒	If greater than the specified range, disconnect the harness connector at the HO2S 2. Verify the HO2S reading is between 35-55 mV. If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the HO2S 2.

5. Connect the test lamp between battery positive voltage and the HO2S 2 sensor input circuit terminal 4.
6. Ignition ON, verify the scan tool HO2S 2 voltage is greater than 1125 mV.

⇒	If less than the specified range, test the signal circuit for an open/high resistance. If the circuit tests normal, replace the PCM.

7. Verify that none of the following conditions exist:

•	Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.

•	Splits, kinks, or improper connections at the vacuum hoses

•	The fuel system operating pressure is incorrect--Refer to Fuel System Diagnosis.

•	The fuel injectors are operating lean or rich. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F).

•	Missing, restricted, or leaking exhaust components--Refer to Exhaust Leakage.

•	Vacuum leaks at the intake manifold, manifold fittings, or throttle body - P0171

•	Faulty crankcase ventilation system--Refer to Crankcase Ventilation System Inspection/Diagnosis.

•	A collapsed or restricted air intake duct and a restricted or dirty air filter - P0172

•	Excessive fuel in the crankcase - P0172

•	The incorrect operation of the evaporative emission control system--Refer to Evaporative Emission Control System Diagnosis.

8. Engine running at normal operating temperature, observe the scan tool HO2S 1 voltage parameters listed below while commanding the fuel injector rich or lean with the scan tool.

•	Command the Fuel Injector Modification Commanded State to +25 percent -- The HO2S 1 voltage should decrease to less than 3.0 volts.

•	Command the Fuel Injector Modification Commanded State to -11 percent -- The HO2S 1 voltage parameter should increase to greater than 3.35 volts.

•	Command the Fuel Injector Modification Commanded State to 0 percent -- The HO2S 1 voltage parameter should fluctuate between 3.2-3.4 volts.

⇒	If the scan tool HO2S 1 voltage parameter was not within the specified ranges, replace the HO2S 1.

9. If all the tests are normal, replace the HO2S 2.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.