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 100-900 mV. The powertrain control module (PCM) monitors this voltage and determines by the exhaust oxygen content whether the air/fuel mixture is rich or lean. If the voltage input at the PCM is about 100 mV, the HO2S indicates a lean air/fuel mixture. If the voltage input is about 900 mV, the HO2S indicates a rich air/fuel mixture. The PCM constantly monitors the HO2S signal during closed-loop operation and compensates for an incorrect air/fuel mixture by changing the fuel-injector pulse width. A DTC P0134 will be set if the HO2S 1 voltage remains outside the active voltage range for an extended period of time.

### Conditions for Running the DTC

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

• | The fuel tank level is above 25 percent.

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

• | The barometric pressure is more than 75 kPa.

• | The engine is running at a steady speed.

### Conditions for Setting the DTC

The HO2S voltage remains less than equal to 300 mV for at least 10 seconds.

OR

The HO2S voltage remains more than or equal to 600 mV for at least 10 seconds.

### 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 without detecting a fault.

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

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

### Diagnostic Aids

Check for any of the following conditions:

DTC P0134 is likely to set during city driving. Avoid operating the vehicle on the highway when validating a DTC P0134.

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 scan tool indicates YES, check for DTC P0134 under Last Test Failed. If DTC P0134 is not listed, the condition is intermittent, and the HO2S system diagnostic is indicating OK at this time.

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

• | A faulty HO2S.An HO2S that has a faulty internal ground circuit or an open in the signal circuit will be inactive.A contaminated HO2S.Silicon contamination of the HO2S can cause a sluggish signal voltage to be indicated.

• | Silicon contamination is indicated by a powdery white deposit on the portion of the HO2S that is exposed to the exhaust stream. Before replacing the HO2S, determine and correct the cause of the contamination. A HO2S that becomes coated with carbon or with other combustion materials may not become active, and may set DTC P0134.

• | Visually inspect the HO2S for this type of contamination. After repairing the cause of the contamination, clean the HO2S by running a warmedup engine at 1,800-2,200 RPM for 2 to 3 minutes. A faulty electrical connection to the PCM

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

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 P0134 cannot be duplicated, the Freeze Frame data can be useful in determining the vehicle 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 some basic checks, and to save the applicable freeze frame data on the scan tool. This creates an electronic copy of the data that was taken when the fault occurred. The scan tool saves the information for later reference.

2. This step determines if the fault is present.The vehicle must be at operating temperature.

3. This step checks if the malfunction that caused the DT P0134 is still present. Drive the vehicle under the specified conditions to verify that the fault is present. The DTC P0134 diagnostic can be monitored under the MIL/System Status selection of System Information on the scan tool . The HO2S test displays a YES status in order to indicate that the purge system diagnostic is completed.If the test displays a YES, check for a DTC P0134 in the Last Test Failed screen of the scan tool. If DTC P0134 is not displayed, the HO2S system diagnostic has run and passed.This indicates that a malfunction was not present this time. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the system diagnostics that are being performed. Remember that the MIL/System Status tests only indicate that the test has run.This will not indicate if the test passed or failed. The Last Test Failed screen must be checked for related DTCs in order to determine the results of the diagnostic test that was 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 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 other DTCs set besides a P0131, P0132, P0133, P0136, P0171, P0172? | -- | 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 2000 RPM for 1 minute.
4. Operate the vehicle within the parameters that were specified in the first 3 items that were listed under Conditions for Setting the DTC.
5. Monitor the HO2S 1 voltage with a scan tool.

Does the HO2S 1 voltage switch below the specified value and 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 on the scan tool.
3. Operate the vehicle within the Freeze Frame conditions that are specified or until the HO2S test is complete. Refer to the Test Description.

Does the HO2S 1 voltage switch below the specified value 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 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. 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 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 on the scan tool, and road--test the vehicle within the Freeze Frame conditions which set the DTC.
2. Review the scan tool data, and check the data for DTCs. The repair is complete if no DTCs are displayed.

Are any DTCs displayed on the scan tool?

--

Go to the Applicable DTC Table

System OK