P0171

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) monitors the oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made to the fuel delivery will be indicated by the long and short term fuel trim values. Ideal fuel trim values are around 128 (0%). Whenever the oxygen sensor signal is indicating a lean condition, the PCM will add fuel resulting in fuel trim values above 128 (0 to 100%). Whenever 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. Whenever the exhaust emissions reach an excessive level due to a lean condition, a fuel trim DTC is set.

Conditions For Setting The DTC
- When one or more of the following DTCs are not set P0106, P0107, P0108, P011 2, P011 3, 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, P1133, P1406, P1441.
- The Fuel Trim Index is above 165.
- The engine coolant temperature is between 60°C (140°F) and 115°C (239°F).
- The engine speed is between 750 RPM and 3400 RPM.
- The vehicle speed is below 70 mph.
- The Throttle Position (TP) angle is below 80%.
- The Manifold Absolute Pressure (MAP) is greater than 28 kPa.
- The Barometric pressure is greater than 70 kPa.
- The Intake Air Temperature (IAT) sensor is between -25°C (-13°F) and 115°C (239°F).
- The above conditions are present for greater than 4 seconds.
- EVAP is not occurring.

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.

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.
- DTCs can be cleared by using a scan tool.

Diagnostic Aids
- Check for a poor connection at the PCM. Inspect 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. 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.

DTC P0171 - Fuel Trim System Lean (1 Of 2):

DTC P0171 - Fuel Trim System Lean (2 Of 2):

Test Description
Numbers below refer to the step numbers on the Diagnostic Table.
1. The OBD System Check prompts the Technician to complete some basic checks and store the Freeze Frame Data and the Failure records on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. This information on the scan tool can be referred to later.
5. Visually/physically checking items which may cause a lean condition may determine the cause of the DTC being set.
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.