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), P0090 Fuel Rail Pressure (FRP) Regulator Control Circuit Malfunction, and illuminate the MIL (malfunction indicator lamp). Note: Open Circuit means that a component or sensor in a system is deprived of the reference voltage it needs to work.
Causes of open circuits are many and varied, but for the most part, this type of code is caused by broken wiring, poor connections across electrical connectors or previously repaired wiring, loss of ground that prevents current flow, blown fuses, defective relays, faulty switches, or any of a host of other issues and problems that prevents the flow of current through wiring.