OBD 2 diagnostic code P0171 indicates that bank one is too lean. An engine is said to run" lean" when the air-to-fuel ratio goes above the idealratio 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.
Bank one refers to the side of the engine where the number one cylinder is located. An engine with cylinders in a V configuration like a V-6 or V-8 will have two banks. Therefore, a code P0171 indicates that the bank with the number one cylinder is not getting enough fuel. In V type engines, a code for bank one lean is often accompanied with a lean code from the companion bank. So, depending on the engine and circumstances involved with a lean code, an engine might have two different 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. Typical four cylinders don't have a second bank, so, any lean code will log as a lean problem on bank one.
Lean codes can indicate vacuum leaks, fuel delivery problems like clogged fuel filters or injectors. Sometimes, even the sensors that tell the computer how the engine is running may be a fault. It's very common to solve lean codes on some vehicles, many Ford models in particular, simply by cleaning the 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 could skew the data getting back to the computer and cause engine drivability problems, reduced power and even lean codes.