DTC P0171: Bank 1 Fuel Injection System Too Lean Or DTC P0174: Bank 2 Fuel Injection System Too Lean : Description
WARNING: This page is about a different car, the 2003 Nissan Pathfinder. However, it is still accessible from the selected car via links, so may be relevant.
With the air/fuel mixture ratio self-learning control, the actual mixture can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the front Heated Oxygen Sensor (HO2S). The ECM calculates the necessary compensation to correct the offset between the actual and theoretical ratios.
When the amount of compensation is extremely large (the actual mixture is too lean), the ECM determines that the fuel injection system is malfunctioning and turns on the MIL (2-trip detection logic).