For 1990-2009 cars

### Circuit Description

To provide the best possible combination of driveability, fuel economy, and emission control, a Closed Loop air/fuel metering system is used. While in Closed Loop, the powertrain control module (PCM) monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltages. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0 percent. If the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0 percent. If a rich condition is detected, the fuel trim values will be below 0 percent, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, the PCM will set DTC P0171. The PCMs maximum authority to control long term fuel trim allows a range between -29-+22 percent. The PCMs maximum authority to control short term fuel trim allows a range between -27 to +27 percent. The PCM monitors fuel trim under various operating conditions, fuel trim cells, before determining the status of the fuel trim diagnostic. The fuel trim cells are as follows:

Cell | Purge ON | Purge OFF
--- | --- | ---
Idle (Cell 0) | X | --
Decel (Cell 1) | X | --
Normal (Cell 2) | X | --
Accel (Cell 3) | -- | --
High Flow (Cell 4) | -- | --
Long Term F.T. average more than +21%
Short Term F.T. more than +2%

The vehicle may have to be operated in all of the above conditions marked by an X, before the PCM determines fuel trim status. The amount of fuel trim deviates from 0 percent in each cell and the amount that each cell is used directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.

### Conditions for Running the DTC

• | No MAF, MAP, TP, ECT, IAT, CKP, CAM Sensor, Misfire, Fuel Injector, VSS, EST, HO2S, EGR, or EVAP DTCs are set.

• | Engine coolant temperature is between 20°C (68°F) and 110°C (230°F).

• | Intake air temperature is between -18°C (0°F) and 70°C (158°F).

• | Manifold absolute pressure is between 18 kPa and 80 kPa.

• | Throttle angle is steady and less than 90 percent.

• | Vehicle speed is less than 132 km/h (82 mph).

• | Engine speed is between 600 and 4000 RPM.

• | Baro is more than 70 kPa.

• | Air flow is between 3 gm/s and 150 gm/s.

### Conditions for Setting the DTC

• | Long term fuel trim average is more than 21 percent.

• | Short term fuel trim is more than 2 percent.

### 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/Failure Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has 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 a scan tool.

### Diagnostic Aids

Inspect for the following conditions:

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

• | Poor PCM to engine block grounds-- Refer to Ground Distribution Schematics in Wiring Systems.

• | Fuel pressure low--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 P0171 may be set. Refer to Fuel System Diagnosis .

• | Fuel injectors faulty--Refer to Fuel Injector Balance Test .

• | Vacuum leaks--Inspect for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system. Refer to Visual/Physical Inspection in Symptoms .

• | Exhaust leaks--An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Inspect for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust System Inspection in Exhaust Systems.

• | Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace the 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 Diagnosis .

### Test Description

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

1. DTCs other than P0171 may indicate a condition present which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0171.

1. If the DTC P0171 test passes while the Fail Records conditions are being duplicated, the lean condition is intermittent. Refer to Diagnostic Aids or Symptoms for additional information on diagnosing intermittent problems.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 | Are any DTCs set other than P0171? | -- | Go to applicable DTCs | Go to Step 3
3 |

1. Start and run the vehicle in Closed Loop.
2. Observe Long Term F.T. and Short Term F.T. display on the scan tool data list.

Are the displayed values at or near the specified values?

• | Long Term F.T. is the first specified value.

• | Short Term F.T. is the second specified value.

+21%

+2%

Go to Step 5

Go to Step 4

4 |

1. Review and record the scan tool Fail Records data.
2. Clear the DTC P0171 and operate the vehicle to duplicate the Fail Records conditions.
3. Monitor the scan tool Specific DTC info for DTC P0171 while operating the vehicle to duplicate the Fail Records conditions.
4. Continue operating the vehicle until the DTC P0171 test runs and note test result.

Does the scan tool indicate DTC P0171 failed this ignition?

--

Go to Step 5

Go to Diagnostic Aids

5 |

1. Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections.
2. Repair any problem found. Refer to Visual/Physical inspections in Symptoms .

Did your inspection reveal a problem requiring repair?

--

Go to Step 22

Go to Step 6

6 |

1. Visually and physically inspect the crankcase ventilation valve for proper installation.
2. Repair any problem found. Refer to Crankcase Ventilation System Description .

Did the inspection reveal a problem requiring repair?

--

Go to Step 22

Go to Step 7

7 |

1. Inspect the throttle body inlet screen for damage or for the presence of foreign objects which may partially block the airflow sample through the MAF sensor.
2. Correct any problem that is found.

Did your inspection of the throttle body reveal a condition requiring repair?

--

Go to Step 22

Go to Step 8

8 |

1. Start the engine.
2. Observe the idle quality.

Is a high or unsteady idle being experienced?

--

Go to Step 9

Go to Step 11

9 | With the engine idling, observe the IAC display on the scan tool. Is the displayed value more than the specified value? | 5 | Go to Step 11 | Go to Step 10
10 |

1. Visually and physically inspect the throttle body, intake manifold, EGR valve and the EGR feed pipes for vacuum leaks.
2. Repair any vacuum leaks as necessary.

Did your inspection reveal a vacuum leak?

--

Go to Step 22

Go to Step 11

11 |

1. Perform Idle Air Control System Diagnosis. Refer to Idle Air Control (IAC) System Diagnosis .
2. Correct any IAC problem.

Did you find and correct the condition?

--

Go to Step 22

Go to Step 12

12 | Test the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was the fuel contaminated? | -- | Go to Step 22 | Go to Step 13
13 |

1. Visually and physically inspect the PCM injector grounds, power grounds and sensor grounds to ensure that they are clean, tight, and in their proper locations.
2. If a faulty ground condition is present, correct it as necessary. Refer to Wiring Repairs in Wiring Systems.

Did your inspection reveal a condition requiring repair?

--

Go to Step 22

Go to Step 14

14 |

1. Disconnect the MAF sensor electrical connector.
2. Operate the vehicle in Closed Loop while monitoring the Short Term F.T. displayed on the scan tool.

Does Short Term F.T. value decrease to near the specified value?

0%

Go to Step 21

Go to Step 15

15 | Perform the procedure in Fuel System Pressure Test and repair fuel system problem if necessary. Refer to Fuel System Pressure Test . Did the Fuel System Pressure Test isolate a condition requiring repair? | -- | Go to Step 22 | Go to Step 16
16 | Perform the EVAP Control System Diagnosis and repair EVAP system problem if necessary. Refer to Evaporative Emission Control System Diagnosis . Did you find and correct the condition? | -- | Go to Step 22 | Go to Step 17
17 |

1. Visually and physically inspect the intake manifold, injector O-rings, EGR adapter, EGR valve, and the EGR feed pipes for vacuum leaks.
2. Repair any problem that is found.

Did your inspection reveal a problem?

--

Go to Step 22

Go to Step 18

18 |

1. Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware.
2. Correct any problem found. Refer to Visual/Physical inspection in Symptoms .

Did your inspection reveal a problem?

--

Go to Step 22

Go to Step 19

19 | Perform the Injector Balance Test and correct any problem found. Refer to Fuel Injector Balance Test . Did the Injector Balance Test isolate a problem? | -- | Go to Step 22 | Go to Step 20
20 | Visually and physically inspect the HO2S to ensure that it is installed securely and that the HO2S pigtail and wiring harness are not contacting the exhaust or otherwise damaged. Did your inspection reveal a problem? | -- | Go to Step 22 | Go to Diagnostic Aids
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the replacement? | -- | Go to Step 22 | --
22 |

1. Review and record the scan tool Fail Records data.
2. Clear the DTC P0171 and operate the vehicle to duplicate the Fail Records conditions.
3. Monitor the scan tool Specific DTC info for DTC P0171 while operating the vehicle to duplicate the Fail Records conditions.
4. Continue operating the vehicle until the DTC P0171 test runs and note test result.

Does the scan tool indicate DTC P0171 failed this ignition?

--

Go to Step 2

System OK