For 1990-2009 cars

### System Description

Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content in the exhaust stream. The HO2S must reach operating temperature to provide an accurate voltage signal. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature. The powertrain control module (PCM) supplies the HO2S with a reference, or bias, voltage of about 450 mV. When the engine is first started the PCM operates in open loop, ignoring the HO2S voltage signal. Once the HO2S reaches operating temperatures and Closed Loop is achieved, the HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage. High HO2S voltage indicates a rich exhaust stream. Low HO2S voltage indicates a lean exhaust stream.

If the PCM detects an HO2S voltage that stays below a specified value, DTC P1171 will set.

### Conditions for Running the DTC

• | DTCs P0122, P0123, P0131, P0132, P0133, P0134 are not set.

• | The ECT Sensor parameter is more than 20°C (68°F).

• | The Engine Run Time parameter is more than 20 seconds.

• | The Loop Status parameter is closed.

• | The Power Enrichment parameter is active.

### Conditions for Setting the DTC

The PCM detects that the HO2S 1 parameter is less than 300 mV for 5 seconds.

### Action Taken When the DTC Sets

• | The control module stores the DTC information into memory when the diagnostic runs and fails.

• | The malfunction indicator lamp (MIL) will not illuminate.

• | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.

• | The driver information center, if equipped, may display a message.

### Conditions for Clearing the DTC

• | A current DTC Last Test Failed clears when the diagnostic runs and passes.

• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.

• | Clear the DTC with a scan tool.

### Diagnostic Aids

• | A restricted fuel filter can supply adequate amounts of fuel at idle, but may not be able to supply enough fuel during heavy acceleration. Water or alcohol in the fuel may cause low HO2S 1 voltage during acceleration.

• | High resistance in the ignition coil control circuits may cause this condition.

• | MAP sensor-High resistance in the MAP sensor circuits may cause this condition.

• | Inspect for faulty or plugged fuel injectors.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. When the system is operating correctly, the HO2S 1 voltage should toggle above and below the bias voltage. You may need to operate the vehicle within the Freeze Frame conditions and Conditions for Setting the DTC in order to duplicate the malfunction that was detected by the PCM.

2. The specified value is what is measured on a correctly operating system.

Step | Action | Values | 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 |

1. Operate the engine at normal operating temperature.
2. Operate the engine above 1,200 RPM for 2 minutes.
3. Observe the HO2S 1 voltage parameter with a scan tool.

Does the scan tool indicate that the heated oxygen sensor (HO2S) 1 voltage is less than the specified values?

300 mV

Go to Step 4

Go to Step 3

3 |

1. Observe the Freeze Frame/Failure Records for this DTC.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4 |

1. Turn OFF the ignition.
2. Disconnect the HO2S sensor.
3. Turn ON the ignition, with the engine OFF.
4. Measure the voltage of the HO2S 1 high signal circuit on the powertrain control module (PCM) side with the DMM.

Does the voltage measure within the specified value?

350-550 mV

Go to Step 6

Go to Step 5

5 | Test the HO2S 1 high signal circuit for a short to ground or a short to the HO2S 1 low signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 8
6 |

1. The HO2S 1 is detecting a lean condition or may be contaminated. Inspect for the following conditions:

• | HO2S connector water intrusion

• | Silicon-contaminated HO2S

• | An exhaust leak between the HO2S 1 and the engine. Refer to Exhaust Leakage in Engine Exhaust.

• | Vacuum leaks

• | Incorrect fuel pressure

• | Refer to Fuel System Diagnosis .

• | Lean fuel injectors

• | Refer to Fuel Injector Balance Test with Tech 2 .

2. Repair any of the above or similar engine conditions as necessary.

Did you find and correct the condition?

--

Go to Step 11

Go to Step 7

7 | Inspect for poor connections at the harness connector of the HO2S 1 sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 9
8 | Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 10
9 | Important: Before replacing the HO2S determine and remove any source of contamination. Replace the HO2S 1. Refer to Heated Oxygen Sensor 1 Replacement . Did you complete the replacement? | -- | Go to Step 11 | --
10 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 11 | --
11 |

1. Clear the DTCs with a scan tool.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 12

12 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK