OBD-II Code P3400 is defined as a Cylinder Deactivation System
In an effort to meet fuel and emissions standards, many manufacturers are utilizing cylinder deactivation systems. This system allows the engine's control module (PCM) to disable select cylinders when additional power is not needed from the engine, such as at idle or when cruising on the highway. If the engine's control module (PCM) determines that there is a fault in the Cylinder Deactivation System, the PCM will set code P3400.