DTC P0171: System Too Lean (Bank 1): General Description

When the engine is started, the ECM monitors fuel related sensors such as MAFS, TPS, ETCS and IATS. The ECM then calculates the amount of fuel that is needed. Once the HO2S is producing a signal, the ECM checks for correct fuel delivery based on the HO2S values (closed loop). The corrections can be seen through the "Short Term Fuel Trim" (STFT). If the HO2S shows that the system is running a little lean, the ECM will add fuel. The ideal fuel trim value is around 0%. When adding fuel, the value will move up and the value will lower when moving lean. The STFT can move quickly but it cannot add or remove large amounts of fuel. The ECM tries to keep the STFT near the middle of the scale; so the system can make corrections quickly in both directions. To do this, the ECM will add or remove fuel through another calculation called "Long Term Fuel Trim" (LTFT). By adding fuel through LTFT, the STFT will move back toward the no correction value.