For 1990-2009 cars

### Circuit Description

The DTC P0131 HO2S Circuit Low Voltage Sensor 1 diagnostic monitors the output of the fuel control HO2S. The heated oxygen sensor (HO2S) produces a voltage that varies between 100 mV and 900 mV under normal operating conditions. The powertrain control module (PCM) monitors this voltage and determines if the exhaust is indicating a lean or a rich fuel mixture. If the voltage input at the PCM is about 100 mV the fuel mixture is lean, and if the voltage input is about 900 mV the fuel mixture is rich. The PCM constantly monitors the HO2S signal during Closed Loop operation and compensates for a rich or lean condition by decreasing or increasing the fuel injector pulse width as necessary. If the HO2S 1 voltage remains excessively low for an extended period of time, DTC P0131 will be set.

Air Fuel Mixture | Exhaust O2 Content | HO2S 1 Output | Fuel System Response
--- | --- | --- | ---
Lean Mixture | High Oxygen | Low Voltage | Rich Command
Rich Mixture | Low Oxygen | High Voltage | Lean Command

### Conditions for Running the DTC

• | Barometric pressure (BARO) is more than 75 kPa.

• | Engine coolant temperature (ECT) is -8 to +110°C (18-230°F).

• | Intake air temperature (IAT) is more than -8°C (+18°F) at engine start up.

• | Vehicle speed is more than 55 km/h (35 mph) for 2 minutes and then idled for 1 minute.

### Conditions for Setting the DTC

The maximum average voltage is less than 600 mV and the minimum average voltage is less than 300 mV.

OR

The PCM internal voltage check of the HO2S 1 circuit while idling indicates a higher than expected voltage drop.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after 2 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 will be stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The PCM turns OFF the MIL after 3 consecutive trips without a fault.

• | The PCM clears a history DTC after 40 consecutive warm-up cycles without a fault.

• | Perform the scan tool clear DTC Information function 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.

Check for any of the following conditions:

• | If any DTCs other than fuel trim (P0171, P0172), misfire (P0300-P0304), EGR (P0400), and HO2S 1 (P0131-P0134) are set, diagnosis those DTCs first.

• | A misrouted heated oxygen sensor wiring. The sensor pigtail may be mispositioned and contacting the exhaust system.

• | The correct fuel pressure. The fuel system can go lean if the fuel pressure is too low, and DTC P0131 may set.

• | Lean fuel injectors

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

• | Exhaust leaks. An exhaust leak may cause outside air to be pulled into the exhaust gas stream and past the HO2S, causing DTC P0131 to set. Check for exhaust leaks near the HO2S.

• | A shorted HO2S. If an HO2S is shorting to ground, the HO2S reading displayed on the scan tool will be less than 0.1 volts. Water contamination of the HO2S can cause a low HO2S voltage to be indicated. Inspect whether DTC P0131 occurs when driving through standing water or when operating the vehicle in wet driving conditions. Replace the HO2S if the sensor is adversely affected by moisture.

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

• | Fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. Too much alcohol in the fuel can also cause a lean exhaust indication.

If a DTC P0131 is intermittent, driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. Refer to Inspection/Maintenance Heated Oxygen Sensor/Oxygen Sensor System Set .

An intermittent malfunction may be caused by a fault in the HO2S sensor circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

If the 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 Diagnostic System Check-Engine Controls 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. The vehicle must be at operating temperature.

3. This step checks whether the malfunction that caused the DTC P0131 is still present. After performing the scan tool clear DTC information function, the DTC P0131 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0131 appears in the Not Ran Since Code Cleared list, the P0131 diagnostic has not yet run. When the DTC P0131 does not appear in the Not Ran Since Code Cleared list, the P0131 diagnostic has run. If the MIL is NOT illuminated and there is no Pending DTC Status in DTC Information, the P0131 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. The Not Ran Since Code Cleared list only indicates whether of not the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, 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 measure 80 mV or less.

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

6. This step checks the HO2S 1 signal circuit. The DMM is used as a low current voltage source, taking the place of the HO2S. This test verifies whether the signal wire is good, and whether the PCM is able to receive and process a signal from the HO2S.

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 may cause ignition positive voltage to be shorted to the signal circuit.

8. Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0131 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P01131 appears in the Not Ran Since Code Cleared list, the DTC P0131 diagnostic has not yet run. When the DTC P0131 does not appear in the Not Ran Since Code Cleared list, the DTC P0131 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0131 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 | Are any HO2S 1 DTCs set other than DTC P0131 and DTC P0132? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Step 3
3 |

1. Start the engine.
2. Run the engine until the engine reaches a 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 range?

300 mV - 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 P0131 diagnostic test has run. Refer to the Test Description.

Does the HO2S 1 voltage switch below and above the specified range?

300 mV - 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.

Is the voltage within the specified range?

0 mV - 80 mV

Go to Step 6

Go to Step 10

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

1. Use a digital multimeter (DMM) and select the Millivolt DC scale.
2. Connect one lead of the DMM to B+.
3. Connect the other lead of the DMM to the HO2S 1 signal circuit.
4. Observe the HO2S 1 voltage parameter on the scan tool.

Is the voltage near the specified value?

1000 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 1 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. Turn OFF the ignition.
2. Remove the DOME fuse for 30 seconds.
3. Install the DOME fuse.
4. Reset the radio clock time. Refer to Radio/Audio System Description and Operation in Entertainment.
5. Start the engine.
6. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or until the DTC P0131 diagnostic test has run.

Does the DTC run and pass?

--

Go to Step 14

Go to Step 2

14 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK