For 1990-2009 cars

Schematic Reference: Engine Controls Schematics

### 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 about 100 mV the HO2S is indicating a lean air/fuel mixture. When the voltage input is about 900 mV the HO2S is indicating rich air/fuel mixture. The PCM constantly monitors the HO2S signal during closed loop operation and compensates for the rich or the lean condition by decreasing or increasing 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

• | Barometric pressure is greater than 75 kPa.

• | Intake air temperature is between -10°C to 70°C (14°F to 158°F).

• | Fuel tank level is greater than 25%.

• | Engine coolant temperature is greater than 80°C (176°F).

• | Vehicle speed is above 50 km/h (30 mph) for 2 minutes and then idled for 2 minutes.

• | Engine is running at steady speed.

### Conditions for Setting the DTC

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 the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after three consecutively passing trips without a fault present.

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

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

### Diagnostic Aids

A DTC P0131 is more likely to set during city driving. Avoid operating the vehicle on the highway when validating a DTC P0131.

Clear the DTCs. Road test the vehicle while monitoring the HO2S test in the MIL/System Status selection under System Information on the scan tool. When the HO2S test indicates complete with a YES status, check for a DTC P0131 under Last Test Failed. If there is no DTC P0131 indicated, the DTC is intermittent and the HO2S system diagnostic is indicating OK at this time.

Check for any of the following conditions:

• | Misrouted heated oxygen sensor wiring. The sensor pigtail may be misrouted and contacting the exhaust system 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 and a DTC P0131 may be set. Refer to Fuel System Diagnosis .

| A lean (incorrectly calibrated) fuel injector.

• | Vacuum leaks. Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, the throttle body, and the crankcase ventilation system.

• | Exhaust leaks near the HO2S. An exhaust leak may cause outside air to be pulled into the exhaust gas stream and flowed past the HO2S. The outside air may cause a DTC P0131 to set.

• | A shorted heated oxygen sensor (HO2S). An HO2S that is shorted to ground will display under 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 more frequently when driving through standing water or when operating the vehicle in wet driving conditions can indicate a water intrusion. Replace the HO2S if it is adversely affected by moisture.

• | A contaminated HO2S. Silicon contamination of the HO2S can cause a low signal voltage to be indicated. Silicon contamination is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. Before replacing the HO2S determine the cause and correct the condition.

• | 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 Alcohol/Contaminants-in-Fuel Diagnosis .

• | A faulty 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 components for any of the following conditions:

• | Backed out terminals.

• | Improper mating of terminals.

• | Broken electrical connector locks.

• | Improperly formed or damaged terminals.

• | Faulty terminal to wire connections.

• | Physical damage to the wiring harness.

• | A broken wire inside the insulation.

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

If a DTC P0131 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was 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 some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

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

3. This step checks whether the malfunction that caused the DTC P0131 is still present. Driving the vehicle under these 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 System Information. When the HO2S test displays a YES status (indicating that the purge system diagnostic is completed) check for a DTC P0131 in the Last Test Failed screen of the scan tool. If there is no DTC P0131 displayed the HO2S system diagnostic has run and passed, indicating that no malfunction was present this time. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the system diagnostics being performed. Do not forget that 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 outcome of the diagnostic test 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 or 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 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 System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Are any other HO2S 1 DTCs set besides a P0131 and a P0132? | -- | Go to the Applicable DTC Table | Go to Step 3
3 |

1. Start the engine.
2. Run the engine 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 switch below and above the specified value?

300 mV to 600 mV

Go to Step 4

Go to Step 5

4 |

1. Turn ON the ignition, leaving the engine OFF.
2. Perform the scan tool 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 switch below and above the specified value?

300 mV to 600 mV

Fault not Present-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 mV to 35 mV

Go to Step 6

Go to Step 10

6 | Probe the HO2S 1 ground circuit (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 DMM 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 or 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. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | Go to Step 13 | --
13 |

1. Perform the scan tool Clear DTC Information function and road test the vehicle within the Freeze Frame conditions that set the DTC.
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