DTC P0172 Or DTC DTC P0175: Description
WARNING: This page is about a different car, the 2002 Nissan Sentra. 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 HO2S1 (bank 1 or bank 2). The ECM calculates the necessary compensation to correct the offset between the actual and theoretical ratios. See Figure .
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).