P0171
CIRCUIT DESCRIPTION
Fuel Trim Cells:
To provide the best possible combination of driveability, fuel economy, and emissions control, a closed loop air/fuel metering system is used. While in closed loop, the PCM monitors the HO2S 1 signals and adjusts the fuel delivery based upon the HO2S signal voltages. A change made to the fuel delivery will be indicated by the long term and short term FT 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 of 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 on bank 1, the PCM will set DTC P0171. The PCMs maximum authority to control long term fuel trim allows a range between -20 percent and +20 percent. The PCMs 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 shown on the illustration.
The vehicle may have to be operated in all of the above conditions 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.
CONDITIONS FOR RUNNING THE DTC
^ None of the following active DTCs are present:
- Throttle Position (TP) sensor
- Evaporative Emissions (EVAP) system
- Misfire
- Intake Air Temperature (IAT) sensor
- Manifold Absolute Pressure (MAP) sensor
- Fuel injector circuit
- EGR pintle position
- Engine Coolant Temperature (ECT) sensor
- Crankshaft Position (CKP) sensor
- Mass Air Flow (MAF) sensor
- Vehicle Speed Sensor (VSS)
- Camshaft Position (CMP) sensor
- Heated Oxygen Sensor (HO2S)
- Electronic Spark Timing (EST)
^ 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 85 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 and 4000 RPM.
^ The BARO is more than 70 kPa.
^ The mass air flow is between 3 gm/s and 150 gm/s.
CONDITIONS FOR SETTING THE DTC
^ The long term fuel trim is at or near the maximum authority of +20 percent.
^ The short term fuel trim is at or near the maximum authority of +20 percent.
^ All of the conditions described above have been met in the Fuel Trim Cells.
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 and Failure Records data.
^ If the misfire is determined to be catalyst damaging, the PCM will flash the MIL.
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 a scan tool.
DIAGNOSTIC AIDS
Check for the following conditions:
^ The HO2S pigtail may be routed incorrectly and contacting the exhaust system.
^ Ensure that the PCM to engine block grounds are clean, tight, and in the proper locations. Refer to Ignition Switch and PCM Signals in Engine Controls Schematics.
^ The fuel system will go lean if the fuel pressure is too low. The PCM can compensate for some decrease. However, if the fuel pressure is too low, a DTC P0171 may be set. Refer to Fuel System Pressure Test. Fuel System Pressure Test
^ The fuel injectors, refer to Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Coil Test - Engine Coolant Temperature (ECT) Outside 10-35 Degrees C (50-95 Degrees F). Component Tests and General Diagnostics Component Tests and General Diagnostics
^ Check for the following vacuum leak conditions:
- Any disconnected or damaged vacuum hoses, refer to Emission Hose Routing Diagram.
- An incorrectly installed or malfunctioning crankcase ventilation valve, refer to Crankcase Ventilation System Inspection. Positive Crankcase Ventilation
- Any vacuum leaks at the throttle body, the EGR valve, and the intake manifold mounting surfaces
^ Check for exhaust leaks that may cause a false lean condition to be indicated. An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Refer to Exhaust Leakage (Not OBD II) in Engine Exhaust. Testing and Inspection
^ Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace MAF sensor.
^ 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. Component Tests and General Diagnostics
^ A faulty connection at the PCM-Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper terminal tension.
^ 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
Steps 1 - 4:
Steps 5 - 15:
Steps 16 - 23:
The numbers below refer to the step numbers on the diagnostic table.
2. 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 DTC P0171.
4. If DTC P0171 test passes while the Failure Records conditions are being duplicated, the lean condition is intermittent. Refer to Diagnostic Aids for additional information on diagnosing intermittent conditions.