P0171
P0171 Bank 1 System Too LeanSystem 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 powertrain control module (PCM)/engine control module (ECM) monitors the oxygen sensor (O2S) signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with the scan tool. Ideal fuel trim values are around 128 (0%). If the oxygen sensor signal is indicating a lean condition, the PCM/ECM will add fuel resulting in fuel trim values above 128 (0% to 100%). If a rich condition is detected, the fuel trim values will be below 128 (0% to -100%), indicating that the PCM/ECM is reducing the amount of fuel delivered. If exhaust emissions reach an excessive level due to a lean or rich condition, a fuel trim Diagnostic Trouble Code (DTC) is set.
Conditions for Setting the DTC
- No OBD II intrusive tests active.
- DTC(s) P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0201, P0202, P0203, P0204, P0336, P0337, P0341, P0342, P0351, P0352, P0402, P0404, P0406, P0506, P0507, P1404, P1130, P1133, P1134, P1171, P1441, and P1627 no test.
- Throttle Position (TP) is less than 95%.
- Engine speed is greater than 400 rpm but less than 6000 rpm.
- Barometric Pressure (BARO) is greater than 72.3 kPa.
- Coolant temperature is between 165 °F (74 °C) and 234 °F (112 °C).
- Manifold Absolute Pressure (MAP) is between 24 and 100 kPa.
- Intake Air Temperature (IAT) is between -40 °C (-40 °F) and 120-C (248 °F).
- Airflow is between 0.008 and 200 g/sec.
- Vehicle speed is less than 101 mph (163 kph).
- System is in closed loop.
- Adaptive index is ready.
- System voltage is greater than 11 volts.
Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignitions in which the diagnostic runs with the fault active.
- The PCM/ECM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history DTC is stored.
Conditions for Clearing the MIL/DTC
- The MIL will turn off alter three consecutive ignition cycles in which the diagnostic runs without a fault within the freeze frame conditions that the DTC failed.
- A history DTC will clear after 40 consecutive warm-up cycles without a fault.
- DTC(s) can be cleared by using the scan tool.
Diagnostic Aids
IMPORTANT: After repairs use the scan tool Fuel Trim Reset function to reset long term fuel trim to 128 (0%).
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.
Map sensor - An output that causes the PCM/ECM 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/ECM 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 a poor O2S or MAP sensor connection at the PCM/ECM. Inspect the harness connectors for the following conditions:
- Backed-out terminals
- Improper mating
- Broken locks
- Improperly formed
- Damaged terminals
- Poor terminal-to-wire connection
Check for a damaged harness. Inspect wiring harness for damage. If the harness appears to be OK. observe the O2S 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.
Check the brake power booster check valve for possible leaks.
Step 1 - 5:
Step 6 - 14:
Step 15 - 17:
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
5. Visually/physically checking items which may cause a lean condition may determine the cause of the DTC being set and save diagnosis time.
9. A vacuum leak can change the Fuel Trim Index and set DTC P0171. This step checks the intake manifold for vacuum leaks.
11. Contaminants in fuel, such as alcohol or water, can create a lean condition setting DTC P0171. Checking for these contaminants could identify the malfunction.
17. If no faults have been found at this point and no additional DTCs were set, refer to "Diagnostic Aids" for additional checks and information.