What does it mean?
Diesel engines and gasoline direct injection (GDI) engines have something in common: They inject fuel directly into the cylinder on compression. To make this possible, both systems utilize two fuel pumps. A supply pump, mounted in or near the fuel tank, supplies fuel to the secondary pump and, in the case of GDI engines, the port fuel injectors. This primary pressure usually maxes out around 75 psi.
The secondary mechanical fuel pump, mounted on the engine, bumps that pressure many times higher, GDI engines up to 2,500 psi and diesel engines over 4,000 psi. This high fuel pressure is necessary, to overcome cylinder pressures, as direct injections systems need to inject fuel on the compression stroke, unlike low-pressure port fuel injection systems, which injects fuel into a partial vacuum.
At the same time, such high fuel pressures can be inherently dangerous and destructive, which makes fuel pressure regulation critical. Within specifications, high fuel pressure is necessary and expected. Low fuel pressure would result in poor engine performance and fuel economy. On the other hand, a spike in fuel pressure could actually damage the system. The engine control module (ECM) controls fuel rail pressure (FRP) by modulating the FRP regulator, using feedback from the FRP sensor.
If the ECM detects a problem in the FRP regulator circuit, it will set a DTC (diagnostic trouble code), P0091 Fuel Rail Pressure (FRP) Regulator Control Circuit Low, and illuminate the MIL (malfunction indicator lamp). Note: Circuit Low Input codes are often the result of low battery voltages (that can have many possible causes), bad connections across electrical connectors or previously repaired wiring, as well as corrosion in electrical connectors.
Other possible causes of low input voltages include poor installation of aftermarket components, poor quality aftermarket components like fuses, relays, and switches, and modification of the electrical system that could include the use of conductors that are not rated for use in a particular application. However, poor connections often result in high resistances in some parts of the circuit, which is why it is important to perform resistance and continuity checks during the diagnostic procedure.