For 1990-2009 cars

### Circuit Description

The PCM supplies a bias voltage of about 450 mV between the HO2S signal and low circuits. When measured with a 10 megohm digital voltmeter, this may display as low as 320 mV. The oxygen sensor varies the voltage within a range of about 1000 mV when the exhaust is rich, down through about 10 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 injector pulse width as necessary. If the Bank 2 HO2S 1 voltage remains excessively low for an extended period of time, DTC P0151 will be set.

### Conditions for Setting the DTC

• | No active transmission, TP sensor, EVAP system, EGR, misfire, IAT sensor, MAP sensor, fuel trim, fuel injector circuit, EVAP solenoid ODM, ECT sensor, or MAF sensor DTC(s) present.

• | Closed loop commanded air/fuel ratio is between 14.5 to 1 and 14.8 to 1.

• | Throttle angle is between 3% and 40%.

• | Bank 2 HO2S 1 signal voltage remains below 175 mV during normal closed loop operation.

OR

Bank 2 HO2S 1 signal voltage remains below 600 mV during power enrichment mode fuel control operation.

Either condition must be present for at least 5 seconds.

### Action Taken When the DTC Sets

• | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.

• | The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.

• | The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

• | The DTC can be cleared by using the scan tool.

### Diagnostic Aids

Check for the following conditions:

• | Heated oxygen sensor wiring damaged - The sensor pigtail may be mispositioned and contacting the exhaust system.

• | Poor PCM to engine grounds

• | Fuel pressure low - A condition which causes a lean exhaust can cause DTC P0151 to set. The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0151 may be set. Refer to Fuel System Pressure Test .

• | Fuel injector(s) faulty - Refer to Fuel Injector Balance Test .

• | Vacuum leaks - Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system.

• | Exhaust leaks - An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the DTC P0151 to set. Check for exhaust leaks near the HO2S 2 sensor.

• | Disconnect the MAF sensor and see if the condition is corrected. If so, replace MAF sensor. Refer to Mass Airflow Sensor Replacement .

• | Fuel contamination - Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Test .

### Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table:

1. DTC P0151 failing during power enrichment fuel mode may indicate a condition described in the Diagnostic Aids above. If the DTC P0151 test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated. Review the Fail Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.

2. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to the Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Install scan tool
2. Engine at operating temperature
3. Operate vehicle within parameters specified under DTC Will Set When criteria included in Diagnostic Support.
4. Using a scan tool, monitor Bank 2 HO2S 1 voltage.

Does the Bank 2 HO2S 1 voltage remain below the specified value?

300 mV

Go to Step 4

Go to Step 3

3 |

1. Turn ON the ignition switch.
2. Review and record scan tool Fail Records data and note parameters.
3. Operate vehicle within Fail Records conditions as noted.
4. Using a scan tool, monitor Specific DTC info for DTC P0151 until the DTC P0151 test runs.

Does scan tool indicate DTC P0151 failed this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4 |

1. Disconnect Bank 2 HO2S 1.
2. Jumper the Bank 1 HO2S 3 low circuit (PCM side) to ground.

Does scan tool indicate Bank 2 HO2S 1 voltage near the specified value?

450 mV

Gor to Diagnostic Aids

Go to Step 5

5 |

1. Turn OFF the ignition switch.
2. Disconnect the PCM.
3. Check the Bank 2 HO2S 1 signal circuit for a short to ground or a short to the sensor ground circuit.

Is the Bank 2 HO2S 1 signal circuit shorted?

--

Go to Step 6

Go to Step 7

6 | Repair the Bank 2 HO2S 1 signal circuit. Refer to Heated Oxygen Sensor (HO2S) Repair in Electrical Diagnosis. Is the action complete? | -- | Go to Step 8 | --
7 | Replace the PCM. Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | Go to Step 8 | --
8 |

1. Turn ON the ignition switch.
2. Review and record scan tool Fail Records data.
3. Clear DTCs
4. Operate vehicle within Fail Records conditions as noted.
5. Using a scan tool, monitor Specific DTC info for DTC P0151 until the DTC P0151 test runs.

Does the scan tool indicate DTC P0151 failed this ignition?

--

Go to Step 2

System OK