What does it mean?
OBD II fault code P0256 is a generic code that is defined as “Injection Pump Fuel Metering Control “B” Malfunction (Cam/Rotor/Injector)”, and is set on diesel applications when the PCM (Powertrain Control Module) detects that the volume of fuel being delivered to the injectors does not agree with the volume that is expected to be delivered given the current operating conditions. Note that this code only applies to diesel powered applications.
Like gasoline engines, diesel applications use sophisticated control mechanisms and controllers to control both the volume of fuel injected, and to manage the timing of the injection event.
Using input data from the throttle pedal position sensor, the PCM calculates the volume of fuel required to suit current operating conditions; a Fuel Control Actuator (aka “fuel quantity adjuster”) that is located either in or near the main injection pump then delivers the required volume of fuel to the injectors.
However, the PCM needs to “know” how much fuel is actually being delivered, so it uses a dedicated sensor, known as the Fuel Rack Position sensor, to monitor the position of the Fuel Control Actuator, and to signal this information back to the PCM. This sensor is almost invariably of the pressure sensitive type, which means that its resistance changes as the changing fuel pressure/volume act on it.
Thus, as the pressure/volume of the fuel being passed by the Fuel Control Actuator changes as operating conditions change, the PCM interprets these changes as being directly proportional to the volume and pressure of the fuel being injected into the cylinders. This input data allows the PCM to control the fuel pressure/volume, pulse width, and injection timing very precisely.
Note that the PCM is preprogrammed with complex algorithms that calculate the pressure/volume of fuel that is required to suit any given set of operating conditions.
These requirements are continuously compared to input data from various engine and fuel system sensors (in addition to input data from the Fuel Rack Position sensor) by the PCM, and when it detects that the expected fuel pressure/volume does not match the actual fuel pressure/volume under any given set of operating conditions, it will set code P0256, and illuminate a warning light.
Note that the PCM also monitors the position of the Fuel Control Actuator via the Fuel Rack Position sensor during initial KOEO (Key-On-Engine-Off) self-tests. The image below shows a typical Fuel Control Actuator, such as might be found on almost any common-rail diesel application.
Note that unlike older applications that have a high-pressure line leading from the pump to each injector, injection pumps on common-rail systems have only one outlet that connects to the fuel rail that supplies all the electronically-controlled injectors with fuel, hence the term, “common-rail”.