| Table 1: | LEAN |
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 pre-catalyst HO2S signals and adjusts fuel delivery based upon these 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 percent; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values 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 this DTC. The PCM s maximum authority to control long term fuel trim allows a range between -20 percent and +20 percent. The PCM s maximum authority to control short term fuel trim allows a range between -20 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 as follows:
Cell | Purge ON | Purge OFF |
|---|---|---|
Idle (Cell 0) | X | -- |
Decel (Cell 1) | -- | -- |
Normal (Cell 2) | X | X |
Accel (Cell 3) | X | X |
High Flow (Cell 4) | X | -- |
The vehicle may have to be operated in all of the above conditions (marked with X) before the PCM determines 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 to complete the diagnostic.
Important: Some conditions can cause multiple cells to indicate lean.
Below are some examples of why each cell could indicate a lean condition in each cell.
| • | Idle (Purge ON) - An EVAP system leak, manifold vacuum leak, severe lack of fuel supply, fuel contamination or an exhaust leak. |
| • | Idle (Purge OFF) - A manifold vacuum leak, severe lack of fuel supply, fuel contamination or an exhaust leak. |
| • | Decel (Purge ON) - An EVAP system leak, manifold vacuum leak, fuel contamination or an exhaust leak. |
| • | Decel (Purge OFF) - A manifold vacuum leak, fuel contamination or an exhaust leak. |
| • | Normal (Purge ON or OFF) - A lack of fuel supply, fuel contamination or an exhaust leak. |
| • | Accel (Purge ON or OFF) - A lack of fuel supply, fuel contamination or an exhaust leak. |
| • | High Flow (Purge ON or OFF) - A lack of fuel supply, fuel contamination or an exhaust leak. |
Below are some reasons for a lack of fuel supply.
| • | A faulty or restricted fuel injector. |
| • | A restricted fuel line. |
| • | A faulty or restricted fuel pump. |
| • | A faulty or restricted fuel regulator. |
| • | A restricted fuel filter. |
Conditions for Running the DTC
| • | No VSS, EST, CKP, CAM, TP, Misfire, Fuel injector, MAF, O2, MAP, EGR, EVAP, ECT or IAT 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 percent. |
| • | Vehicle speed is less than 132 km/h (82 mph). |
| • | Engine speed is between 600 and 4000 RPM. |
| • | Baro is more than 70 kPa. |
| • | Air flow is between 3 gm/s and 150 gm/s. |
Conditions for Setting the DTC
| • | Long term fuel trim average is more than +17 percent and the short term fuel trim average is more than +2 percent of the indicated cells in the Fuel Trim Cell Table. |
| • | All of the conditions described above have been met in the Fuel Trim Cells for more than 1 second. |
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:
| • | Fuel pressure/flow low - The system will go lean and add fuel during high fuel demand. The Accel and the High Flow cells will increase to more than 0 percent if this condition is present. The PCM can compensate for some decrease. However, if fuel pressure is too low, both DTC P0171 and P0174 may be set. Refer to Fuel System Pressure Test . |
| • | Fuel injectors faulty - The system will go lean and add fuel if a fuel injector is not suppling enough fuel. All cells could increase to more than 0 percent depending on the severity of the problem. In most cases high fuel demand cells such as Accel and High Flow will be more likely to show the problem. Refer to Fuel Injector Balance Test . |
| • | Vacuum leaks - Inspect for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system and crankcase ventilation system. The Idle cells (Purge ON or OFF) will increase to more than 0 percent if this condition is present. |
| • | EVAP system leaks - Inspect for disconnected or damaged EVAP lines and hoses. The Idle cell with (purge ON) will increase to more than 0 percent if this condition is present. |
| • | 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. Inspect for exhaust leaks that may cause a false lean condition to be indicated. All cells could increase to more than 0 percent depending on the severity of the problem. |
| • | 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 . All cells could increase to more than 0 percent depending on the severity of the problem. |
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.
-
This step determines whether the fault is present. For any test that requires probing the PCM or a component harness connector, use the Connector Test Adapter Kit. Using this kit prevents damage to the harness connector terminals.
-
Using the Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and/or Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass and/or a fail. Operate the vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that you observed. This will isolate when the DTC failed. For an intermittent condition, refer to Symptoms.
-
If both lean DTCs are set, both banks of the engine are operating lean. Inspect the items that would cause both banks to operate lean.
-
A vacuum leak causes DTCs P0171 and P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure the engine oil fill cap is in place and that it is tight. Insure the engine oil dip stick is fully seated.
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||||||||||||
|
Important: If any DTCs are set, except P0171 and P0174 refer to those DTCs before proceeding with this table.
Does the scan tool indicate fuel trim less than the specified value indicated? | +17% LT | |||||||||||||
Does the scan tool indicate that this DTC failed this ignition? | -- | Go to Diagnostic Aids | ||||||||||||
Is the other Lean DTC also set? | -- | |||||||||||||
Visually/physically inspect the following items:
Did you find any problems in any of the above areas? | -- | |||||||||||||
6 | Visually/physically inspect the following items:
Was a problem found in any of the above areas? | -- | Go to Fuel System Diagnosis | |||||||||||
7 | Repair or replace any faulty items found. Did you complete the repair? | -- | -- | |||||||||||
8 |
Does this DTC reset? | -- | ||||||||||||
9 | Does the scan tool indicate any DTCs that you have not diagnosed? | -- | Go to applicable DTC | System OK |
