The DTC P0004 OBDII code is related to the electrical part of the fuel volume regulator system. The automotive fuel system consists of a lot of components. Among these components, we can highlight the fuel filter, injectors, lines, and fuel pump.
The purpose of the pump is to increase the fuel pressure until it reaches a necessary pressure in the fuel rail for the injectors. These pumps have a high-pressure side and a low-pressure side. This pressure is controlled by a Fuel Volume Regulator (FVR).
If an electrical reading is detected in the controller that is above the set limit, this code will be set. Take into account this code is not commonly seen but appears in most cases on diesel or gasoline direct injection vehicles.