For 1990-2009 cars
Engine Controls Components
Cell 21: HO2S Sensors
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The heated oxygen sensor (HO2S) produces a voltage that varies between 100 mV and 900 mV. The powertrain control module (PCM) monitors this voltage and determines by the exhaust gas oxygen content whether the air/fuel mixture is rich or lean. When the voltage input at the PCM is approximately 100 mV, the HO2S is indicating a lean air/fuel mixture. When the voltage input is approximately 900 mV, the HO2S is indicating rich air/fuel mixture. The PCM constantly monitors the HO2S signal during closed-loop operation.The PCM compensates for a rich condition or for a lean condition by changing the fuel-injector pulse width. A DTC P0131 will be set if the HO2S 1 voltage remains excessively low for an extended period of time.

### Conditions for Running the DTC

• | The barometric pressure is more than 75 kPa.

• | The intake air temperature is between -10°C and +70°C (14°F-158°F).

• | The fuel tank level is more than 25 percent.

• | The engine coolant temperature is above 80°C (176°F).

• | The vehicle speed exceeds 50 km/h (30 mph) for 2 minutes, after which the vehicle idles for 2 minutes.

• | The engine is running at a steady speed.

### Conditions for Setting the DTC

The maximum HO2S voltage is less than 300 mV.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM records the operating conditions at the time that the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the DTC

• | The MIL turns OFF after 3 consecutive trips during which the diagnostic runs and does not detect a fault.

• | A History DTC clears after 40 consecutive warm-up cycles without a fault.

• | Use the Clear DTC Information function, or disconnect the PCM battery feed in order to clear the DTC.

### Diagnostic Aids

DTC P0131normally sets during city driving. Avoid operating the vehicle on a highway when validating DTC P0131.

Clear the DTCs. Road-test the vehicle while monitoring the HO2S test in the MIL/System Status selection of System Information. When the scan tool indicates YES , check for DTC P0131 in the Last Test Failed . If the scan tool does not indicate DTC P0131, the DTC is intermittent, and the HO2S system diagnostic is indicating OK at this time.

Check for any of the following conditions:

• | Any misrouted HO2S wiring.The sensor pigtail may be contacting the exhaust system and causing a short to ground.

• | Check for good PCM-to-engine electrical grounds.

• | Incorrect fuel pressure.The fuel system can go lean if the fuel pressure is too low. Refer to Fuel System Diagnosis .

| An incorrectly calibrated fuel injector

• | Any vacuum leaks.Check for disconnected vacuum hoses and for damaged vacuum hoses.Check for vacuum leaks at the intake manifold, at the throttle body, and at the crankcase ventilation system.Any exhaust leaks near the HO2S.An exhaust leak may cause outside air to be pulled into the exhaust gas stream and past the HO2S.

• | The outside air may cause a DTC P0131 to set. A shorted HO2S.An HO2S shorted to ground measures less than 0.1 volts on the scan tool.

• | Water contamination of the HO2S can cause a low HO2S voltage to be indicated. A DTC P0131 that occurs when driving through standing water or when operating the vehicle in wet driving conditions can indicate water intrusion. Replace the HO2S if the sensor is affected by moisture. A contaminated HO2S-Silicon contamination of the HO2S can cause low signal voltage to be indicated.

• | A powdery white deposit on the portion of the HO2S that is exposed to the exhaust stream indicates silicon contamination . Determine the cause of the condition, and correct the condition before replacing the HO2S . Check for fuel contamination.

• | Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. Refer to . A faulty Alcohol/Contaminants-in-Fuel Diagnosis electrical connection to the PCM.

• | An intermittent malfunction may be caused by a fault in the HO2S 1 electrical circuit.

Inspect the wiring harness and the components for any of the following conditions: Backed out terminals

• | Incorrect mating of terminals

• | Broken electrical connector locks

• | Incorrectly formed terminals or damaged terminals

• | Faulty terminal-to-wire connections

• | Physical damage to the wiring harness

• | A broken wire inside the insulation

• | Corrosion of electrical connections, of splices, or of terminals

• | If DTC P0131 cannot be duplicated, the Freeze Frame data can be useful in determining the operating conditions when the DTC first set.

### Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete basic checks and to store the freeze frame data on the scan tool, if applicable. This creates an electronic copy of the data that is captured when the fault occurred. The information is stored in the scan tool for later reference.

2. This step provides a quick check of the HO2S operation.The vehicle must be at operating temperature.

3. This step determines if the malfunction that caused the DTC P0131 is still present. Driving the vehicle under the conditions will verify that the fault is present. The DTC P0131 diagnostic can be monitored on the scan tool under the MIL/System Status selection of the System Information. When the scan tool displays YES, check for DTC P0131 in Last Test Failed. If DTC P0131 is not listed, the HO2S system diagnostic has run and passed, indicating that a malfunction was not present this time. Any DTCs must be cleared in order to view the current status of the system diagnostics that are being performed. The MIL/System Status tests only indicate that the test has run, not whether the test passed or failed. The Last Test Failed screen must be checked for related DTCs in order to determine the result of the diagnostic test that is involved.

4. This step checks for a voltage on the HO2S signal circuit. When the oxygen sensor is disconnected, the scan tool should display 35 mV or less.

5. This step checks for an open in the HO2S 1 ground circuit.

6. This step checks for an open and for a short in the HO2S 1 signal circuit.

7. A possible cause of a voltage short to the HO2S signal circuit is water. Check the wiring harness and the PCM connectors for water that could cause ignition positive voltage to be shorted to the signal circuit.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On--Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Are any HO2S 1 DTCs set except P0131 and P0132? | -- | Go to the Applicable DTC Table | Go to Step 3
3 |

1. Start the engine.
2. Run the engine in order to attain normal operating temperature.
3. Increase the engine speed to 2,000 RPM for 1 minute.
4. Monitor the HO2S 1 voltage with a scan tool.

Does the HO2S 1 voltage vary from below to above the specified value?

300-600 mV

Go to Step 4

Go to Step 5

4 |

1. Turn ON the ignition, leaving the engine OFF.
2. Perform the Clear DTC Information function.
3. Operate the vehicle within Freeze Frame conditions as specified or until the HO2S test is complete. Refer to the Test Description.

Does the HO2S 1 voltage vary below and above the specified value?

300-600 mV

Go to Diagnostic Aids

Go to Step 5

5 |

1. Disconnect the HO2S 1 electrical connector.
2. Observe the HO2S 1 voltage parameter on the scan tool.

Does the scan tool indicate a HO2S 1 voltage within the specified value?

0-35 mV

Go to Step 6

Go to Step 10

6 | Probe the HO2S 1 ground circuit on the harness side with a test lamp connected to B+. Does the test lamp illuminate? | -- | Go to Step 7 | Go to Step 8
7 |

1. Check the HO2S 1 signal voltage with a DMM.
2. Select the DC voltage scale on the DMM.
3. Connect one DMM lead to B+, and connect the other lead to the HO2S 1 signal circuit.
4. Observe the HO2S 1 voltage parameter on the scan tool.

Does the scan tool indicate a HO2S 1 voltage at the specified value?

1275 mV

Go to Step 11

Go to Step 9

8 |

1. Check for an open in the HO2S 1 ground circuit.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 13

Go to Step 12

9 |

1. Check the HO2S 1 signal circuit for an open and for a short to ground.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 13

Go to Step 12

10 |

1. Check the HO2S 1 signal circuit for a short to voltage.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 13

Go to Step 12

11 | Replace the HO2S 1. Refer to Heated Oxygen Sensor Replacement . Is the action complete? | -- | Go to Step 13 | --
12 | Replace the PCM. Is the action complete? | -- | Go to Step 13 | --
13 |

1. Perform the Clear DTC Information function, and road-test the vehicle within the Freeze Frame conditions.
2. Review the scan tool data and check for DTCs. The repair is complete if no DTCs are stored.

Are any DTCs displayed on the scan tool?

--

Go to the Applicable DTC Table

System OK