Installed at the end of the common rail, the Rail Pressure Sensor (RPS) measures the instantaneous fuel pressure in the common rail by using its diaphragm. Its sensing element (semiconductor device) mounted on the diaphragm converts the fuel pressure to an electric signal, which is used by the ECM to precisely control the injection amount and time. Also, if the actual rail pressure calculated from the sensor output signal differs from the target rail pressure, it controls the rail pressure by using the rail pressure control valve.