In cars with an electronic returnless fuel system, a fuel pressure sensor -- a small electronic device located along a vehicle's fuel rail -- feeds fuel pressure data to the power train control module, or PCM.
Electronic returnless fuel systems reduce evaporative emissions from automobiles by feeding the engine only the precise amount of fuel it needs to run. This way, the engine avoids leaving excess fuel to evaporate in the fuel line.
The fuel pressure sensor senses the amount of fuel in the fuel rail by reading the internal pressure of the rail. It can then send an electronic message to the PCM telling it precisely how much fuel needs to be released into the engine.
In an electronic returnless fuel system, the PCM -- as the vehicle's onboard computer that regulates engine functions -- takes the pressure information delivered to it by the fuel pressure sensor and relays the engine's fuel needs to the fuel pump, which supplies the engine with fuel.