What does it mean?
OBD II fault code P0251 is a generic code that is defined as “Injection pump A, rotor/cam -circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects a loss of continuity, or a general failure in the signal/feedback circuits between itself, the fuel metering control sensor, the fuel quantity actuator, and other sensors involved in determining the fuel pressure and volume. Note that this code applies exclusively to diesel-powered applications.
All common-rail diesel injection systems use a device known as an “electronic fuel control actuator” to meter the amount of fuel that is allowed to enter the actual pumping chamber(s) of a high-pressure diesel fuel pump. This device therefore determines both the volume, and the pressure of the fuel that is ultimately delivered to the fuel rail.
However, the pressure in the fuel rail needs to be controlled within a very narrow range, and while there are various ways in which this is accomplished, most applications use a variable resistance-type pressure sensor to communicate the actual pressure to the PCM to ensure that the actual pressure in the fuel rail closely matches the nominal design pressure.
In a fully functional fuel delivery system, the pressure sensor converts fuel pressure into a signal voltage, which the PCM uses to calculate injector pulse width (measured in milliseconds) and injection timing (measured in degrees before Top Dead Center) to ensure that the correct amount of fuel is injected into the cylinders under all operating conditions.
Thus, the signal voltages of the electronic fuel control actuator and the fuel metering control sensor must agree for the PCM to be able to calculate an appropriate fuel delivery strategy to ensure efficient engine operation under all engine speeds and loads. On some applications, code P0251 will set and a warning light be triggered after a single failure, while on others, several failure cycles are required before the code is set and warning light is illuminated.