And here is the DTC information for the P0171 code from the service manual.

DTC P0171 Fuel Trim System Lean
DTC P0172 Fuel Trim System Rich
Circuit Description

The engine control module (ECM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. The fuel trim cells are based on the engine speed and load.
Conditions for Running the DTC


  • DTCs P0101, P0102,P0103, P0106, P0107, P0108, P0112, P0113, P0201, P0202, P0203, P0204, P0217, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0300, P0301, P0302, P0303, P0304, P0442, P0443, P0496, P0449, P0451, P0452, P0453, P0454, P0455, P0461, P0462, P0463, P0464, P0506, and P0507 are not set.
  • The fuel system is in Closed Loop.
  • The engine speed is between 400-6,100 RPM
  • The engine coolant temperature (ECT) is between -38 to +130°C (-36 to 266°F).
  • The intake air temperature (IAT) sensor is between -20°C to +150°C (-4°F to +302°F).
  • The manifold absolute pressure (MAP) is between 15-255 kPa (2-37 psi).
  • The barometric pressure (BARO) is greater than 70 kPa (10 psi).
  • The engine airflow is between 1-512 g/s.
  • The vehicle speed is less than 140 km/h (87 mph).
  • The fuel level is greater than 10 percent.
  • The DTCs run continuously when the above conditions are met.