For 1990-2009 cars

### Circuit Description

The powertrain control module (PCM) supplies a voltage of about 450 mV between the oxygen sensor (O2S 1) high signal and the low signal circuit. The oxygen sensor (O2S 1) varies the voltage within a range of about 1000 mV if the exhaust is rich, down through about 100 mV if the exhaust is lean.

The sensor is like an open circuit and produces no voltage when below 315°C (600°F). An open sensor circuit or a cold sensor causes an Open Loop operation.

### Conditions For Running The DTC

• | DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0171, P0201, P0202, P0203, P0204, P0300, P0301, P0302, P0303, P0304, P0335, P0440, P0442, P0446, P0506, P0507, P0601, P0602, or P1441 are not set.

• | The engine coolant temperature (ECT) is greater than 70°C (158°F).

• | The battery voltage is greater than 10 volts.

• | The fuel level is greater than 10 percent.

• | The engine run time is greater than 10 seconds.

• | The engine is operating in Closed Loop.

• | The throttle position (TP) angle is between 8-50 percent.

• | The manifold absolute pressure (MAP) is greater than 25 kPa (3.62 psi).

• | The engine has been in the above conditions for 4 seconds.

### Conditions For Setting The DTC

O2S 1 voltage is less than 52 mV for 125 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.

### Diagnostic Aids

• | The fuel pressure--The system will be lean if the pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm. Refer to the Fuel System Diagnosis .

• | The MAP sensor--An output that causes the PCM to sense a lower than normal manifold pressure, or high vacuum, can cause the system to go lean. Disconnecting the MAP sensor will allow the PCM to substitute a fixed, or default, value for the MAP sensor. If the lean condition is gone when the MAP sensor is disconnected, refer to the DTC P0105 for further diagnosis.

• | Fuel contamination--Water, in even small amounts, near the in-tank fuel pump inlet, can be delivered to the injector. The water causes a lean exhaust and can set a DTC P0131.

• | Sensor harness--The O2S 1 sensor pigtail may be mis-positioned and contacting the exhaust manifold.

• | Engine misfire--A misfiring cylinder will result in unburned oxygen in the exhaust, which could cause a DTC P0131. Refer to the DTC P0300 and/or Symptoms - Engine Controls .

• | Cracked oxygen sensor (O2S 1) -- A cracked O2S 1 sensor or poor ground at the O2S 1 sensor could cause a DTC P0131. Refer to Symptoms - Engine Controls .

• | Plugged fuel filter--A plugged fuel filter can cause a lean condition and cause a DTC P0131 to set.

• | Plugged oxygen sensor (O2S 1)--A plugged reference port on the Oxygen Sensor (O2S 1) will indicate a lower then normal voltage output from the O2S 1 sensor.

### Test Description

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

1. This step determines if the DTC P0131 is the result of a hard malfunction or an intermittent condition. 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 powertrain control module (PCM).

#### HO2S Low (Pre)

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. Run the engine until the normal operating temperature is reached.
2. Using the scan tool, monitor the O2S voltage for the sensor that applies to this DTC.

Does the O2S voltage measure less than the value specified?

55 mV

Go to Step 4

Go to Step 3

3 | This DTC is intermittent. Are any additional DTCs stored? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Diagnostic Aids
4 |

1. Turn OFF the ignition.
2. Disconnect the O2S sensor.
3. Jumper the O2S low signal circuit to a good ground.
4. Turn ON the ignition, with the engine OFF.
5. Measure the voltage on the O2S high signal circuit on the PCM side, using a DMM that is connected to a good ground.

Does the voltage measure within the specified value?

351-551 mV

Go to Step 5

Go to Step 6

5 | The O2S may be detecting a lean exhaust condition. Check for one of the following conditions:

• | An exhaust leak between the O2S 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 .

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 7

6 | Test the high signal circuit of the O2S for a short to ground or a short to the low signal circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 9
7 | Inspect for poor connections at the harness connector of the O2S sensor. Refer to Testing for Intermittent Conditions and Poor Connections and Heated Oxygen Sensor Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 10 | Go to Step 8
8 | Replace the O2S. Refer to Oxygen Sensor 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 a 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 Diagnostic Trouble Code (DTC) List | System OK