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 wil 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 -25% and +20%. 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) | -- | -- |
Accel (Cell 3) | X | -- |
High Flow (Cell 4) | -- | -- |
Long Term F T average greater than +19% | ||
Short Term F T greater than +4% | ||
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 60°C (140°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%. |
| • | 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 +19%. |
| • | Short term fuel trim is greater than +4%. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecuitive trip in which the diagnostic has been run and failed. |
| • | If equipped with traction control, the PCM will command the EBTCM via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp. |
| • | 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 - 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 . |
| • | Poor connection at the PCM - Inspect harness connectors for the following conditions: |
| - | Backed out terminals. Refer to Repairing Connector Terminals . |
| - | Improper mating. Refer to Repairing Connector Terminals . |
| - | Broken locks. Refer to Connector Repairs . |
| - | Improperly formed or damaged terminals. Refer to Repairing Connector Terminals . |
| - | Poor terminal to wire connection. Refer to Repairing Connector Terminals . |
| • | 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 . |
| • | Poor connection at PCM - Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection(s). Refer to Intermittents and Poor Connections Diagnosis Repairing Connector Terminals and Connector Repairs . |
| • | Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the affected HO2S display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the fault. Refer to Wiring Repairs . |
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.
Test Description
The numbers below refer to the step numbers on the Diagnostic Table:
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain OBD System Check performed? | -- | ||
Are any DTCs set other than P0171? | -- | Go To Applicable DTCs | ||
3 |
Are the displayed values greater than the specified value? | +19% ( L.T.) +4 (S.T.) | ||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||
5 |
Was a problem found? | -- | ||
6 |
Was a problem found? | -- | ||
7 |
Was a problem found? | -- | ||
8 | Start the engine and note the idle quality. 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 less than the specified value? (Value is displayed in counts). | 5 | ||
10 |
Was a problem found? | -- | ||
11 |
Was a problem found? | -- | ||
12 | Test the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was a problem found? | -- | ||
13 |
Was a problem found? | -- | ||
14 |
Was problem found? | -- | ||
15 |
Was problem found? | -- | ||
16 |
Was a problem found? | -- | ||
17 |
Was a problem found? | -- | ||
18 |
Does S.T Fuel Trim decrease to near the specified value? | 0% | ||
19 |
Was a problem found? | -- | ||
20 |
Was a problem found? | -- | Go to Diagnostic Aids | |
21 | Inspect MAF Sensor screen for foreign objects, debris, or damage. Was a Problem found? | -- | ||
22 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is the action complete? | -- | -- | |
23 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | System OK |
