P0171

Diagnostic Chart:

System Description
A Closed Loop air/fuel metering system is used to provide the best possible combination of driveability, fuel economy, and emission control. While in a Closed Loop operation, the PCM monitors the oxygen sensor signal voltage and adjusts the fuel delivery based on the signal voltage. The long term fuel trim values and the short term fuel trim values can indicate any changes to the fuel delivery. Monitor the scan tool for these changes. The 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 you detect a rich condition, the fuel trim values will be below 128 (0% to -100%), indicating that the PCM is reducing the amount of fuel delivered. If the exhaust emissions reach an excessive level due to a lean or a rich condition, a fuel trim DTC is set.

CONDITIONS FOR SETTING THE DTC
DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0125, P0131, P0132, P0133, P0134, P0200, P0300, P0301, P0302, P0303, P0304, P0325, P0335, P0341, P0342, P0401, P0404, P0405, P0502, P0503, P0506, P0507, P0601, P0602, P1404, 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 550 RPM and 3,400 RPM. (Auto)
^ Engine speed is between 850 RPM and 3,400 RPM. (Man)
^ Vehicle speed is below 72 mph (116 km/h).
^ MAP is above 26 kPa.
^ BARO is greater than 72 kPa.
^ EVAP purge is not occurring.
^ Fuel Trim Index is above 170 (33%).
^ 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 the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers.
^ A history DTC will store.
^ The coolant fan turns ON.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The MIL turns 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 clears after 40 consecutive warm up cycles without a fault.
^ Clear the MIL/DTC(s) with a 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. 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 Diagnosis. 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

^ 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 malfunction
^ Check brake power booster check valve for possible leaks.

TEST DESCRIPTION
The number(s) below refer(s) to the step number(s) on the Diagnostic Table.

1. The Powertrain OBD System Check prompts you to complete some of the 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 captured when the malfunction occurred. The information then stores on the scan tool for later reference.

5. This step indicates that visually/physically checking any items which can cause a lean condition can 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 identity the malfunction.

17. If no malfunctions has been present at this point and no additional DTCs were set, refer to DIAGNOSTIC AIDS for any additional checks and for information.