There are certain driving conditions in which maximum engine power is not needed. In such cases, the cylinder deactivation system or variable displacement system is used. Usually, it deactivates the corresponding cylinders and achieves fuel economy in vehicles having more than 8 cylinders.
In this system, variable valve timing solenoids are used to close the intake and exhaust valves of the deactivated cylinders. In this way, cylinder gases are trapped, and the vibration is reduced. This process allows the best operation of the cylinder. By compressing the exhaust gases, the piston is also pushed back to achieve a higher degree of overall engine balance.
The P3400 OBDII fault code is stored because in a certain situation the Powertrain Control Module(PCM) fails to operate the cylinder deactivation system. It is also stored when the cylinder deactivation system is activated unexpectedly.