DTC P0172: Fuel Injection System Too Rich (Bank 1) Or DTC P0175: Fuel Injection System Too Rich (Bank 2) : 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 Sensors (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 or too rich), the ECM determines that the fuel injection system is malfunctioning and turns on the MIL (2-trip detection logic).