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

### Circuit Description

The powertrain control module (PCM) continuously monitors the heated oxygen sensor (HO2S) activity. The PCM counts the number of times that the a rich to lean and lean to rich response is indicated and adds the amount of time it took to complete all the transitions. The PCM uses this information in order to determine an average time for each transition. A DTC P0133 will set if the average transition (response) time is too slow.

A lean to rich transition is indicated when the HO2S voltage changes from less than 300 mV to greater than 600 mV. A rich to lean transition is indicated when the HO2S voltage changes from more than 600 mV to less than 300 mV. An HO2S that responds too slowly is likely to be faulty and should be replaced.

### 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

The response rate average is greater than 1 second.

OR

The switch cycle average is greater than 5 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 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

If any DTCs other than Fuel Trim (P0171, P0172), Misfire (P0300 to P0304) and HO2S 1 (P0131 to P0134) are set, diagnosis those DTCs first.

An intermittent malfunction may be caused by a fault in the HO2S 1 sensor 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 P0133 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 determines if the fault is present.

3. This step checks for an exhaust leak near the HO2S. Fresh air can enter the exhaust system through the exhaust leak and affect HO2S 1 performance.

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 |

1. Turn ON the ignition, leaving the engine OFF.
2. Use the scan tool clear information function.
3. Operate the vehicle within the Freeze Frame conditions as noted.

Does the scan tool indicate any DTCs besides a DTC P0133?

--

Go to the Applicable DTC Table

Go to Step 3

3 | Visually and physically inspect for the following conditions:

• | Proper installation of the HO2S 1.

• | Corrosion of the electrical terminals.

• | Proper tension of the electrical terminal connections at the HO2S 1 and at the PCM.

• | A damaged wiring harness.

• | A contaminated sensor.

Was a problem found in any of the above areas?

--

Go to Step 6

Go to Step 4

4 | Check the exhaust system for an exhaust leak near the engine. Was an exhaust leak found? | -- | Go to Step 6 | Go to Step 5
5 | Important: Inspect the oxygen sensor for contamination before replacing the sensor. Do not replace a contaminated oxygen sensor until the cause of the contamination is found and corrected. Replace the HO2S 1. Refer to Heated Oxygen Sensor Replacement .

Is the action complete?

--

Go to Step 7

--

6 | Repair exhaust condition as necessary. Is the action complete? | -- | Go to Step 7 | --
7 |

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