Fuel Rail Pressure Sensor
WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
The Fuel Rail Pressure (FRP) sensor is a pressure transducer. The PCM supplies about 5 volts on the FRP sensor reference circuit. The Powertrain Control Module (PCM) also supplies a signal and ground circuit to the FRP sensor. When fuel pressure is high the signal voltage is high. The FRP signal is used by the PCM for fuel trim adjustment.