P0171
Fuel Trim System 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 heated oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by long and short term fuel trim values which can be monitored with the scan tool. Ideal fuel trim values are around 0%; if the heated oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 0%. 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 exhaust emissions reach an excessive level due to a lean or rich condition, a fuel trim DTC is set.
Conditions for Running the DTC
^ Barometric pressure greater than 75 kPa.
^ Fuel tank level greater than 25%.
^ Intake air temperature is between -10°C (14°F) and 50°C (122°F).
Conditions for Setting the DTC
^ Long term and short term fuel trim greater than or equal to 30% for 5 seconds.
^ Short term fuel trim greater than or equal to 20% for 45 seconds.
Action Taken When the DTC Sets
^ The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.
^ The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.
Conditions for Clearing the MIL/DTC
^ The MIL will turn "OFF" after three consecutively passing cycles without a fault present.
^ A History DTC will clear after 40 consecutive warm-up cycles without a fault.
^ DTC(s) can be cleared by using a scan tool or by disconnecting the PCM battery feed.
Diagnostic Aids
Check The Following Conditions:
^ Fuel pressure - System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm.
^ Manifold Absolute Pressure (MAP) sensor - An output that causes the PCM to sense a lower than normal manifold pressure (high vacuum) can cause the system to go lean. Disconnecting the MAP sensor will allow the PCM to substitute a fixed (default) value for the MAP sensor. If the lean condition is gone when the sensor is disconnected, substitute a known good sensor and recheck.
^ Fuel contamination - Water, in even small amounts, near the in-tank fuel pump inlet, can be delivered to the injector. The water causes a lean exhaust and can set DTC P0171.
^ Check for poor HO2S 1 or MAP sensor connection at the PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
^ Check for a damaged harness. Inspect 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 harness related to the engine harness. A change in the display will indicate the location of the fault.
If DTC P0171 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.
If any wiring repairs need to be made, Refer to Repair Procedures in Diagrams.
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. If DTC's other than Fuel Trim (P0171, P0172), Misfire (P0300 to P0304), and Exhaust Gas Recirculation (EGR) (P0400) are set, diagnose those DTC's first.
3. Review Freeze Frame data to determine when the DTC set. Always record this information.
4. Any one of these sensors could cause this DTC to set, check the performance of the MAP sensor, Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor and the Idle switch signal, repair any faulty conditions.
8. Before replacing the fuel injector check the resistance of the fuel injector: 0.5 to 1.5 ohms at 20°C (68°F).