Modern vehicles rely on a fuel pump, usually housed inside the fuel tank, to force fuel out of the tank, through tubes (fuel lines) that run along the underside of the vehicle body, and into the fuel rail on top of the engine. The fuel rail routes the fuel to each of the engine’s fuel injectors, and the injectors spray fuel into the combustion chambers.

Over the years, engineers discovered that allowing the fuel pump to run continuously and at full output caused unnecessary heating of the fuel and increased emissions. It also reduced the service life of the fuel pump. Therefore, vehicles are now equipped with a fuel pump control module, a small computer that alters the output of the fuel pump based on the changing requirements of the engine across a range of conditions.

The fuel pump control module varies the amount of electricity that is supplied to the fuel pump. That way, when the engine requires less fuel (such as when it is idling or driving at low speeds), the module will decrease the voltage to the pump, causing it to slow down and produce less fuel pressure. When the vehicle speeds up, climbs a hill, or any other condition that requires more fuel, the control module raises the voltage to the fuel pump and increases its output to compensate.