For 1990-2009 cars

### Circuit 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 temperature 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 P0131 sets for HO2S bank 1 sensor 1 or DTC P0151 sets for HO2S bank 2 sensor 1.

### Conditions for Running the DTC

#### Lean Test Enable:

• | DTCs P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0200, P1125, P1516 are not set.

• | The Loop Status parameter is closed.

• | The Ignition 1 Signal parameter is between 10-18 volts.

• | The Fuel Tank Level Remaining parameter is more than 10 percent.

• | The TP Indicated Angle parameter is between 3-70 percent more than the value observed at idle.

OR

#### Power Enrichment Test Enable:

• | DTCs P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0200, P1125, P1516 are not set.

• | The Loop Status parameter is closed.

• | The Ignition 1 Signal parameter is between 10-18 volts.

• | The Fuel Tank Level Remaining parameter is more than 10 percent.

• | The Engine Run Time parameter is more than 30 seconds.

• | The Power Enrichment parameter is active for more than 1 second.

### Conditions for Setting the DTC

#### Lean Test:

The PCM detects that the affected HO2S voltage parameter is less than 200 mV for 165 seconds.

OR

#### Power Enrichment Test:

The PCM detects that the affected HO2S voltage parameter is less than 360 mV for 10 seconds.

### Action Taken When the DTC Sets

• | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

• | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

• | The control module commands the Loop Status open.

### Conditions for Clearing the MIL/DTC

• | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.

• | 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 emission related diagnostic.

• | Clear the MIL and the DTC with a scan tool.

### Test Description

The number below refers to the step number on the diagnostic table.

1. If the voltage is varying above and below the specified range, the condition is not present.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics Connector End View Reference: Powertrain Control Module Connector End Views or Engine Controls Connector End Views
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Start the engine.
2. Allow the engine to reach operating temperature. Refer to Scan Tool Data List .
3. Observe the affected HO2S voltage parameter with a scan tool.

Is the HO2S voltage parameter varying above and below the specified range?

300-600 mV

Go to Step 3

Go to Step 4

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 Intermittent Conditions

4 |

1. Turn OFF the ignition.
2. Disconnect the affected heated oxygen sensor (HO2S).
3. Turn ON the ignition, with the engine OFF.
4. Observe the HO2S voltage parameter with a scan tool.

Is the HO2S voltage parameter less than the specified value?

100 mV

Go to Step 6

Go to Step 5

5 |

1. Connect a 3-amp fused jumper wire between the high signal circuit of the HO2S harness connector on the engine harness side and a good ground.
2. Observe the HO2S voltage parameter with a scan tool.

Is the HO2S voltage parameter less than the specified value?

100 mV

Go to Step 7

Go to Step 8

6 | Test the HO2S high signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 9
7 | Test the HO2S low signal circuit for a short to the HO2S heater low control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 10
8 | Test the HO2S high signal circuit for an open or for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 15 | Go to Step 12
9 | Test the HO2S high signal circuit for a short to the following circuits:

• | HO2S low signal circuit

• | HO2S heater low control circuit

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 12

10 |

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

Notice: Refer to Silicon Contamination of Heated Oxygen Sensors Notice in the Preface section.

• | A silicon contaminated HO2S

• | Any water intrusion into the HO2S connector

• | An exhaust leak between the HO2S and the engine

• | Any vacuum leaks

• | An incorrect fuel pressure--Refer to Fuel System Diagnosis .

• | Any lean fuel injectors--Refer to Fuel Injector Balance Test with Tech 2 .

• | An inaccurate mass air flow (MAF) sensor--Refer to Scan Tool Data List .

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

Did you find and correct the condition?

--

Go to Step 15

Go to Step 11

11 | Test for shorted terminals and for poor connections at the HO2S. 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 15 | Go to Step 13
12 | Test for shorted terminals and for poor connections at the powertrain control module (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 15 | Go to Step 14
13 | Replace the affected HO2S. Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 or Heated Oxygen Sensor Replacement - Bank 2 Sensor 1 . Did you complete the replacement? | -- | Go to Step 15 | --
14 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 15 | --
15 |

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 16

16 | 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