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 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%; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, 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 -30% and +24%. The PCMs maximum authority to control short term fuel trim allows a range between -27% and +27%. 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) | -- | -- |
L.T. Fuel Trim average greater than +24% | ||
S.T. Fuel Trim greater than +27% | ||
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% 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 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 15 kPa and 85 kPa. |
| • | Throttle angle is steady and less than 90%. |
| • | Vehicle speed is less than 132 km/h (82 mph). |
| • | Engine speed is between 600 and 4000 RPM. |
| • | Baro is greater than 70 kPa. |
| • | Air flow is between 3 gm/s and 150 gm/s. |
Conditions for Setting the DTC
| • | Long term fuel trim is at or near maximum authority of 24%. |
| • | Short term fuel trim is greater than 2%. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. |
| • | The PCM writes the conditions to the Freeze Frame and updates the Failure Records. |
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
Check 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 Power and Grounding Component Views . |
| • | 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 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. 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. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust System Inspection . |
| • | Disconnect the MAF sensor and see if the lean 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 Diagnosis . |
Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.
| • | Poor terminal connection. |
| Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Intermittents and Poor Connections Diagnosis , and Connector Repairs Wiring Systems. |
| • | Damaged harness. |
| Inspect the wiring harness for damage. If the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems. |
| • | Inspect the PCM and the engine grounds for clean and secure connections. |
If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.
Test Description
The numbers below refer to the step numbers on the Diagnostic Table:
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain on Board Diagnostic System Check performed? | -- | ||||||
Are any DTCs set other than P0171? | -- | Go to the applicable DTCs | ||||||
3 |
Are the displayed values at or near the specified values?
| +16% +20% | ||||||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||||||
5 | Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections and repair any problem found. Refer to Testing for Continuity Intermittents and Poor Connections Diagnosis Repairing Connector Terminals and Connector Repairs . Did your inspection reveal a problem requiring repair? | -- | ||||||
6 | Visually and physically inspect the crankcase ventilation valve for proper installation and repair any problem found. Refer to Crankcase Ventilation System Description . Did inspection reveal a problem requiring repair? | -- | ||||||
7 |
Did your inspection of the throttle body reveal a condition requiring repair? | -- | ||||||
8 |
Is a high or unsteady idle being experienced? | -- | ||||||
9 | With the engine idling, observe the IDLE AIR CONTROL display on the scan tool. Is the displayed value greater than the specified value? (Value is displayed in counts). | 5 | ||||||
10 |
Did your inspection reveal a vacuum leak? | -- | ||||||
11 |
Did the Idle Air Control System Diagnosis isolate a problem requiring repair? | -- | ||||||
12 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was the fuel contaminated? | -- | ||||||
13 |
Did your inspection reveal a condition requiring repair? | -- | ||||||
14 |
Does S.T. Fuel Trim value decrease to near the specified value? | 0% | ||||||
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? | -- | ||||||
16 | Perform the EVAP Control System Diagnosis, and repair EVAP system problem if necessary. Refer to Evaporative Emission Control System Diagnosis . Did EVAP Control System Diagnosis isolate a problem? | -- | ||||||
17 |
Did your inspection reveal a problem? | -- | ||||||
18 | Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware and correct any problem found. Did your inspection reveal a problem? | -- | ||||||
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? | -- | ||||||
20 |
Did your inspection reveal a problem? | -- | Go to Diagnostic Aids | |||||
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is the action complete? | -- | -- | |||||
22 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | System OK |
