For 1990-2009 cars

### Circuit Description

The powertrain control module (PCM) supplies a bias voltage of approximately 450 mV between the heated oxygen sensor (HO2S) high signal and the low reference circuits. When measured with a 10-megaohm digital voltmeter (DVM), this may display as low as 350 mV. The oxygen sensor varies the voltage from approximately 1000 mV when the exhaust is rich, to approximately 100 mV when exhaust is lean. The PCM constantly monitors the HO2S signal during Closed Loop operation and compensates for a rich or lean condition by decreasing or increasing the injector pulse width as necessary. If the HO2S 1 voltage remains excessively low for an extended time, DTC P0131 will set.

### Conditions for Running the DTC

There are no active DTCs of any of the following types present

• | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0201-P0206, P0410, P0440, P0442, P0443, P0446, P0449, or P1441 are not set.

• | The system voltage is more than 9 volts.

• | The Closed Loop commanded air-to-fuel ratio is between 12-16.5.

• | The throttle angle is between 3-35 percent .

### Conditions for Setting the DTC

• | The HO2S 1 signal voltage remains below 175 mV during normal Closed Loop operation.

| OR

• | The HO2S 1 signal voltage remains below 600 mV during Power Enrichment Mode fuel control operation.

• | The above conditions are present for up to 50 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.

### 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 step number on the diagnostic table.

1. In order to determine if the engine is lean during power enrichment, operate the vehicle under higher throttle, or higher acceleration, conditions. Monitor the scan tool HO2S voltage and the power enrichment parameter at the higher acceleration. A lean condition will cause the HO2S voltage to be less than 598 mV during power enrichment.

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 engine until the normal operating temperature is reached.
2. With a scan tool, observe the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage less than the value specified?

175 mV

Go to Step 4

Go to Step 3

3 | The condition that set this DTC is not present. This DTC may have been set by one of the following conditions:

• | A signal wire intermittently shorted to engine ground or to the HO2S low reference circuit

• | HO2S connector water intrusion

• | An exhaust leak between the HO2S and the engine

• | An intermittent lean engine condition resulting from one of the following causes:

- | Vacuum leaks--Refer to Emission Hose Routing Diagram .

- | An incorrect or defective positive crankcase ventilation (PCV) valve or system

- | Incorrect fuel pressure

- | Lean fuel injectors--Refer to Fuel Injector Balance Test with Tech 2 .

- | An inaccurate mass air flow (MAF) sensor

• | A lean engine condition during Power Enrichment operation

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

Did you find and correct the condition?

--

Go to Step 10

Go to Intermittent Conditions

4 |

1. Turn OFF the ignition.
2. Disconnect the HO2S connector for the sensor that applies to this DTC.
3. Jumper the HO2S low reference circuit terminal on the engine harness side to a known good ground.
4. Turn ON the ignition, with the engine OFF.
5. With a scan tool, observe the HO2S voltage for the sensor that applies to this DTC.

Is the HO2S voltage within the specified value range?

351-551 mV

Go to Step 5

Go to Step 6

5 | The HO2S may be detecting a lean exhaust condition. Check for one of the following conditions:

• | HO2S connector water intrusion

• | An exhaust leak between the HO2S and the engine

• | Vacuum leaks

• | An incorrect or defective PCV valve or system

• | Incorrect fuel pressure

• | Lean fuel injectors

• | An inaccurate MAF sensor

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

Did you find and correct the condition?

--

Go to Step 10

Go to Step 8

6 |

1. Turn OFF the ignition.
2. Remove the jumper from the HO2S low reference circuit terminal.
3. Disconnect the PCM connector that contains the HO2S high signal circuit.
4. Use a DMM in order to measure and record the resistance between the HO2S high signal circuit and a ground.
5. Measure and record the resistance between the HO2S high signal circuit and the HO2S low reference circuit.

Is the infinite resistance measured on both tests?

--

Go to Step 9

Go to Step 7

7 | Repair the HO2S high signal circuit shorted to ground or shorted to the HO2S low reference circuit . Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 10 | --
8 | Replace the HO2S. Refer to Heated Oxygen Sensor 1 Replacement . Did you complete the replacement? | -- | Go to Step 10 | --
9 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 10 | --
10 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 11

Go to Step 2

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