P0171

DTC P0171 Bank 1 System Too Lean

System 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 1) 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 O2S 1 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 intrusive tests active.
- DTC(s) P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0201, P0202, P0203, P0204, P0336, P0337, P0341, P0342, P0351, P0352, P0402, P0404, P0406, P0443, P0506, P0507, P1404, P1130, P1133, P1134, P1404, P1441, and P1627 not set.
- The throttle position (TP) is less than 95%.
- The engine speed is greater than 400 rpm but less than 6000 rpm.
- The barometric pressure (BARO) is greater than 72.3 kPa.
- The coolant temperature is 50 °C (122 °F).
- The manifold absolute pressure (MAP) is between 24 and 100 kPa.
- The airflow is between 0.008 and 200 g/sec.
- The vehicle speed is less than 80 mph (128 kph).
- The system is in closed loop.
- The adaptive index is ready.
- The canister purge is enabled.
- The 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 jails. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history DTC is stored.
- The coolant fan turns ON.

Conditions for Clearing the MIL/DTC
- The MIL will turn off after 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
- The 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 the long term fuel trim to 128(0%).

Fuel pressure - The system will be lean if the 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 poor O2S 1 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

Inspect the wiring harness for damage. If the harness appears to be OK, observe the O2S 1 display on the scan tool while moving the connectors and the 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.

Steps 1 - 5:

Steps 6 - 15:

Steps 16 - 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 to 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.