A fuel pressure sensor is a device that reads the pressure in the fuel rail and transmits the information to the engine control unit. The fuel pressure sensor, or FPS, is fitted onto the common rail. It reads pressure with both positive and negative values.
The function of the engine control unit, or ECU, is to control the supply of fuel from the high pressure pump to the fuel rail. This is made possible by the fuel pressure control valve. The FPS sends a signal to the ECU whenever pressure in the fuel rail drops. The high-pressure pump then restores the pressure.