|Code||Fault Location||Probable Cause|
|P0251|| Injection pump A, rotor/cam -circuit malfunction |
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|Wiring, injection pump, ECM|
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What Does Code P0251 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.
Where is the P0251 sensor located?
The image above shows a simplified schematic of a typical fuel pressure control system. Note the location of the fuel control actuator on the high-pressure pump, and the location of the pressure sensor on the fuel rail. Note also that although this particular system employs an electric lift pump in the fuel tank, not all applications are so equipped.
On most applications, the principal sensors/components that make up the injection pump metering control circuit “A” is known as the Fuel Rack Position (FRP) and Fuel Quantity Actuator (FQA) respectively, and are usually mounted alongside the high-pressure injection pump. In some applications, these components are located inside the high-pressure injection pump. Refer to the manual for the application to determine the exact location of all components and wiring to prevent a misdiagnosis.
What are the common causes of code P0251 ?
Some common causes of code P0251 could include the following-
- Damaged, shorted, disconnected, or corroded wiring and/or connectors
- Defective Fuel Rack Position (FRP) sensor
- Defective Fuel Quantity Actuator (FQA)
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced