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 voltage. 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%, 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 PCM's maximum authority to control long term fuel trim allows a range between -23 percent and +16 percent. The PCM's maximum authority to control short term fuel trim allows a range between -11 percent and +20 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 weighted (assigned a percentage of importance in determining total fuel trim status for the diagnostic) as follows:
| • | Idle, purge commanded (Cell 10 - approximately 10 percent). |
| • | Deceleration, purge commanded (Cell 1- approximately 15 percent). |
| • | Normal driving, purge commanded (Cell 2- approximately 25 percent). |
| • | Normal driving, no purge commanded (Cell 6- approximately 45 percent). |
| • | Idle, no purge commanded (Cell 9- approximately 5 percent). |
The vehicle may have to be operated in all of the above conditions before the PCM determines fuel trim status. The amount that fuel trim deviates from 0 percent in each cell and the amount that each cell is weighted 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 VSS, TP sensor, EVAP system, HO2S, Misfire, IAT sensor, MAP sensor, Idle speed, Fuel Injector(s), IC/Bypass, EGR Pintle, EGR Flow, ECT sensor CKP sensor CMP sensor, or MAF sensor DTC(s) 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). |
| • | BARO is greater than 70 kPa. |
| • | Manifold absolute pressure is between 15 kPa and 85 kPa. |
| • | Throttle angle is less than 90 percent. |
| • | Vehicle speed is less than 82 mph. |
| • | Mass air flow is between 3 g/s and 150 g/s. |
| • | Engine speed between 600 and 4000 RPM. |
Conditions for Setting the DTC
| • | Long term fuel trim is at or near maximum authority of +16 percent. |
| • | Short term fuel trim is at or near maximum authority of +20 percent. |
| • | All conditions have been met in Fuel Trim Cells 1, 2, 6, 9 and/or 10. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the 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 Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF 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 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: Ensure that the PCM grounds are clean, tight, and in the proper locations. Refer to Ground Distribution Schematics in Wiring Systems. |
| • | Fuel pressure: 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 Pressure Test . |
| • | Vacuum leaks: Check for the following conditions: |
| - | Disconnected or damaged vacuum hoses. |
| - | Refer to Emission Hose Routing Diagram . |
| - | Incorrectly installed or malfunctioning crankcase ventilation valve. |
| - | Refer to Crankcase Ventilation System Inspection . |
| - | Vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces. |
| • | 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 Leakage - Not OBD II . |
| • | Mass Air Flow sensor: 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 the PCM: Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. |
| • | Damaged harness: Inspect the wiring harness for damage. |
| • | If the harness appears to be OK, observe the HO2S 1 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 malfunction. |
Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
DTCs other than P0171 may indicate that a condition is 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.
-
If the DTC P0171 test passes while the Fail Records conditions are being duplicated, the lean condition is intermittent.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Are any DTCs set other than P0171? | -- | Diagnose the other DTCs first. | ||
3 |
Are the displayed values greater than the specified value? | +15% | ||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||
5 |
Was a problem found? | -- | ||
6 |
Was problem found? | -- | ||
7 |
Refer to Throttle Body Assembly Replacement . 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 greater than the specified value? (Value is displayed in counts). | 5 | ||
10 |
Was a problem found? | -- | ||
11 |
Was a problem found? | -- | ||
12 | Check 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 a problem found? | -- | ||
16 |
Was a problem found? | -- | ||
17 |
Does S.T Fuel Trim decrease to near the specified value? | -- | ||
18 |
Was a problem found? | 0% | ||
19 |
Was a problem found? | -- | ||
20 | Inspect MAF Sensor screen for foreign objects, debris, or damage. Was a Problem found? | -- | Go to Diagnostic Aids | |
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is action complete? | -- | -- | |
22 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | System OK |
