OBD 2 diagnostic code P0174 indicates that bank two is too lean.  An engine is said to run" lean" when the air-to-fuel ratio goes above the ideal 14.7 to 1 ratio and not enough fuel is in the mix.  Gasoline engines are always trying to maintain a good ratio of fuel and air, and when the ratio of air goes beyond normal limits, a lean code will set and a service engine soon light will illuminate. 

Most engines use oxygen sensors in the exhaust stream to determine if the system is running rich or lean at any given moment.  The engine management system can make adjustments to the air fuel mixture based on data inputs it receives.  Information such as engine temperature, as well as intake air volume and engine load can be used to determine if more or less fuel needs to be delivered to the engine.

A P0174 lean code for bank two is sometimes accompanied with a lean code for the companion bank number one.  So, depending on the engine and circumstances involved, an engine might have two lean codes if it has two banks. An engine that has two banks, but only one lean code, may help you determine which side of the engine the problem lies.

Lean codes can indicate vacuum leaks, or fuel delivery problems like low fuel pressure, clogged fuel filters or injectors.  Sometimes, even the sensors that report engine data may be at fault.  It's very common to solve lean codes on some vehicles simply by cleaning dirt off the mass air flow sensor.

Dirty mass air flow sensor

Too much filter oil from washable air filters or infrequent air filter changes can lead to dirt build-up on mass airflow sensor elements.  Dirt on the delicate sensor element could skew the data getting back to the computer and cause engine drivability problems, reduced power and even P0174 codes.