Circuit Description
In order to provide the best possible combination of driveability, fuel economy, and emissions control, a closed loop air-to-fuel metering system is used. While in closed loop, the powertrain control module (PCM) monitors the HO2S 1 signal and adjusts the fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the long-term and the short-term fuel trim (FT) values, which can be monitored with a scan tool. The ideal fuel trim values are approximately 0 percent. If the HO2S signals indicate a lean condition, the PCM will add fuel, resulting in fuel trim values that are more than 0 percent. If a rich condition is detected, the fuel trim values will be less than 0 percent, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, DTC P0171 will set .
The maximum authority of the PCM to control long-term FT allows a range between -23 percent and +16 percent. The maximum authority of the PCM to control the short-term FT allows a range between -11 percent and +20 percent. The PCM monitors the fuel trim during various operating conditions before determining the status of the fuel trim diagnostic. The fuel trim cells that are used in order to determine the fuel trim are as follows:
VIN K
| • | Idle, purge commanded |
| • | Deceleration, purge commanded |
| • | Normal driving, purge commanded |
| • | Normal driving, no purge commanded |
| • | Idle, no purge commanded |
VIN 1
| • | Idle, purge commanded |
| • | Normal driving, purge commanded |
| • | High flow driving, purge commanded |
| • | Normal driving, no purge commanded |
| • | Idle, no purge commanded |
You may need to operate the vehicle during all of the above conditions before the PCM determines fuel trim status. The amount that the fuel trim deviates from 0 percent in each cell and the amount that each cell is weighted directly affect whether or not the vehicle must be operated in all of the cells described above in order to complete the diagnostic.
Conditions for Running the DTC
| • | No VSS, TP sensor, EVAP system, HO2S, Misfire, IAT sensor, MAP sensor, Idle speed, Fuel Injectors, IC/Bypass, EGR Pintle, EGR Flow, ECT sensor CKP sensor CMP sensor, or MAF sensor DTCs are set. |
| • | The engine coolant temperature is between 20°C (68°F) and 110°C (230°F). |
| • | The intake air temperature is between -18°C (0°F) and 70°C (158°F). |
| • | The BARO is more than 70 kPa. |
| • | The manifold absolute pressure is between 15 kPa and 85 kPa. |
| • | The throttle angle is less than 90 percent. |
| • | The vehicle speed is less than 82 mph. |
| • | The mass air flow is between 3 g/s and 150 g/s. |
| • | The engine speed is between 600 RPM and 4,000 RPM. |
Conditions for Setting the DTC
| • | The long-term FT is at or near the maximum authority of +16 percent. |
| • | The short-term FT is at or near the maximum authority of +20 percent. |
| • | VIN K - All conditions have been met in Fuel Trim Cells 1, 2, 6, 9 and/or 10. |
| • | VIN 1 - All conditions have been met in Fuel Trim Cells 2, 4, 6, 9 and/or 10. |
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:
| • | The 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 System. |
| • | The fuel pressure--The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if the fuel pressure is too low, a DTC P0171 will set. Refer to Fuel System Pressure Test (VIN K) or to Fuel System Pressure Test (VIN 1) . |
| • | Vacuum leaks--Inspect 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, the EGR valve, and the intake manifold mounting surfaces. |
| • | Exhaust leaks--An exhaust leak may cause the 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 an indication of a false lean condition. Refer to Exhaust Leakage - Not OBD II in Exhaust Systems. |
| • | The sass air flow sensor (MAF)--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--Even small amounts of water , 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 . |
Many situations may lead to an intermittent condition. Perform each inspection or test as directed.
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.
| • | Loose terminal connection |
| - | Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| - | Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair. |
| • | Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
| • | Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair. |
If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.
Test Description
The numbers below refer to the step numbers on the diagnostic table:
-
DTCs other than DTC 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. Refer to Diagnostic Aids for additional information on diagnosing intermittent conditions.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | ||
Are any other DTCs set? | -- | Diagnose applicable DTCs. Go to Diagnostic Trouble Code (DTC) List/Type . | ||
3 |
Are the values more than the specified value? | +15% | ||
Does the DTC reset? | -- | Go to Diagnostic Aids | ||
5 |
Did you find and correct the condition? | -- | ||
6 |
Did you find and correct the condition? | -- | ||
7 |
Did you find and correct the condition? | -- | ||
8 | Start the engine, and note the idle quality. Is the idle high or unsteady? | -- | ||
9 | With the engine idling, observe the Idle Air Control with a scan tool. Does the value measure more than the specified value? The value is displayed in counts. | 5 | ||
10 |
Did you find and correct the condition? | -- | ||
11 |
Did you find and correct the condition? | -- | ||
12 | Test the fuel for excessive water, for alcohol, or for other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Did you find and correct the condition? | -- | ||
13 |
Did you find and correct the condition? | -- | ||
14 |
Did you find and correct the condition? | -- | ||
15 |
Did you find and correct the condition? | -- | ||
16 |
Did you find and correct the condition? | -- | ||
17 |
Did you find and correct the condition? | -- | ||
18 |
Does Short Term FT decrease to near the specified value? | 0% | ||
19 |
Did you find and correct the condition? | -- | ||
20 |
Did you find and correct the condition? | -- | Go to Diagnostic Aids | |
21 | Inspect the MAF Sensor screen for foreign objects, for debris, and for damage. Did you find and correct the condition? | -- | ||
22 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the replacement? | -- | -- | |
23 |
Does the DTC reset? | -- | System OK |
