| Table 1: | DTC P0171 - Fuel Trim System Lean |
Circuit Description
A closed loop air/fuel metering system is used in order to provide the best possible combination of driveability, fuel economy, and emissions control. While in closed loop, the PCM monitors the HO2S 1 signal and adjusts the fuel delivery based on the HO2S signal voltages. A change to the fuel delivery will be indicated by the long and short term fuel trim 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, the PCM will set DTC P0171. The PCM's maximum authority to control long term fuel trim allows a range from -30--+22 percent. The PCM's maximum authority to control the short term fuel trim allows a range from -27--+27 percent. The PCM monitors the fuel trim during various operating and fuel trim cell conditions before determining the status of the fuel trim diagnostic. The fuel trim cells are:
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 FT average more than +20% | ||
Short Term FT more than +3% | ||
The vehicle may have to be operated in all of the above conditions marked by an X before the PCM determines the 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 in order 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. |
| • | 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 manifold absolute pressure is between 15 kPa and 80 kPa. |
| • | The throttle angle is steady and less than 90 percent. |
| • | The vehicle speed is less than 132 km/h (82 mph). |
| • | The engine speed is between 600--4000 RPM. |
| • | The baro is more than 70 kPa. |
| • | The air flow is between 3 gm/s and 150 gm/s. |
Conditions for Setting the DTC
| • | The long term FT average is more than 20 percent. |
| • | The short term FT is more than +3 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
Use the following table to identify the system or component that is suspected:
System/ Component | Condition |
|---|---|
Sensor | HO2S wiring for correct routing, and contamination. MAF sensor for fluctuation and correct operation, TP sensor for correct throttle response. ECT and IAT sensors for a skewed condition. Refer to the affected sensor performance DTC, Temperature Versus Resistance and Harness Routing Views in wiring Systems. |
Engine/PCM Grounds | Loose or corroded connections--Refer to Ground Distribution Schematics in Wiring Systems. |
Fuel System | Low fuel pressure--Refer to Fuel System Pressure Test . |
Fuel Metering | Mechanically stuck, or electrically inoperable fuel injectors-- Refer to Fuel Injector Balance Test . |
Fuel Quality | Fuel for water, alcohol, or dirt contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
Vacuum leaks/ Unmetered air | Vacuum hoses, intake manifold, throttle body, IAC valve, EGR valve, crankcase ventilation, MAF sensor and air intake duct for correct routing and connection--Refer to Visual/Physical Inspection in Symptoms . |
Exhaust System | Exhaust for leaks, dents, or restrictions--Refer to Exhaust System Inspection in Engine Exhaust. |
Also inspect for:
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:
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||
Are any DTCs set other than P0171? | -- | Go to the applicable DTCs | ||||||
3 |
Are the displayed values at or near the specified values?
| +20 % +3 % | ||||||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||||||
5 | Inspect the vacuum hoses for splits, kinks, and improper connections and repair any problem that you find. Refer to Visual/Physical inspections in Symptoms . Did your inspection reveal a problem that requires a repair? | -- | ||||||
6 | Inspect the crankcase ventilation valve for proper installation and repair any problem that you find. Refer to Crankcase Ventilation System Inspection . Did your inspection reveal a problem that requires a repair? | -- | ||||||
7 |
Did your inspection of the throttle body reveal a condition requiring repair? | -- | ||||||
8 |
Is there a high or unsteady idle? | -- | ||||||
9 | With the engine idling, observe the IAC display on the scan tool. Is the displayed value more than the specified value? | 5 Counts | ||||||
10 |
Did your inspection reveal a vacuum leak? | -- | ||||||
11 |
Did the idle air control system diagnosis isolate a problem that requires a repair? | -- | ||||||
12 | Inspect the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was the fuel contaminated? | -- | ||||||
13 | Inspect the PCM injector grounds, the power grounds and the sensor grounds to ensure that they are clean, tight, and in their proper locations. Refer to Wiring Repairs in Wiring Systems. Did your inspection reveal a condition that requires a repair? | -- | ||||||
14 |
Does short term fuel trim value decrease to near the specified value? | 0% | ||||||
15 | Perform the procedure in the fuel system pressure test and repair the fuel system problem if necessary. Refer to Fuel System Pressure Test . Did the fuel system pressure test isolate a condition that requires a repair? | -- | ||||||
16 | Perform the EVAP control system diagnosis and repair an EVAP system problem if necessary. Refer to Evaporative Emission Control System Diagnosis . Did the EVAP canister purge valve system check isolate a problem? | -- | ||||||
17 | Inspect the intake manifold, the injector O-rings, the EGR adapter, the EGR valve and the EGR feed pipes for vacuum leaks. Did you find a problem? | -- | ||||||
18 | Inspect the exhaust manifold for leaks and for loose or missing hardware and correct any problem that you find. Refer to Visual/Physical inspection in Symptoms . Did you find a problem? | -- | ||||||
19 | Perform the fuel injector balance test and correct any problem. Refer to Fuel Injector Balance Test . Did the Fuel Injector Balance Test isolate a problem? | -- | ||||||
20 | 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 you find a problem? | -- | Go to Diagnostic Aids | |||||
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the replacement? | -- | -- | |||||
22 |
Does the scan tool indicate that DTC P0171 failed this ignition? | -- | System OK |
