P0171

Diagnostic Chart:

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 PCM monitors the oxygen sensor signal voltage and adjusts fuel delivery based on the 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 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 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 SETTING THE DTC

^ DTC's P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0401, P0502, P0503, P0506, P0507, P0601, P1406, P1133, and P1441, not set.

^ Intake Air Temperature (IAT) is between -25°C (-13°F) and 115°C (175°F).

^ Engine Coolant temperature is between 60°C (140°F) and 115°C (239°F).

^ Engine speed is between 575 RPM and 3600 RPM.

^ Vehicle speed is below 70 mph (113 km/h).

^ MAP is above 38 kPa.

^ BARO is greater than 72 kPa.

^ Evaporative Emissions (EVAP) System purge is not occurring.

^ Fuel Trim Index is above 165 (29 %).

^ Above conditions present for over 4 seconds.

ACTION TAKEN WHEN THE DTC SETS

^ The Malfunction Indicator Lamp (MIL) will illuminate if the fault is active within the same conditions for two out of eighty ignition cycles.

Or

^ The 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 and Failure Records buffers.

^ A history DTC is stored.

^ 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 cycle without a fault.

^ DTC(s) can be cleared by using a scan tool.

DIAGNOSTIC AIDS

^ 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. Refer to Fuel System Diagnosis. System Diagnosis - Fuel System

^ 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. Refer to MAP Sensor Output Check. MAP Sensor Output Diagnosis

^ 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. 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 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.

^ Check brake power booster check valve for possible leaks.

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 and failure records 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 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 fault.

17. If no faults have been found at this point and no additional DTC's were set, refer to Diagnostic Aids for additional checks and information.