|Code||Fault Location||Probable Cause|
|P0256|| Injection pump 8, rotor/cam -circuit malfunction |
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|Wiring, injection pump, ECM|
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What Does Code P0256 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”.
What are the common causes of code P0256 ?
Common causes of code P0256 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Clogged or dirty fuel filter. Note that this will almost always be indicated by a dedicated fuel pressure related code.)
- Defective Fuel Control Actuator
- Defective Fuel Rack Position sensor
- Defective injection pump (Note that this is rare)
- Failed or failing fuel control driver in the PCM. (Note that this even more rare than injection pump failures, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P0256 ?
Common symptoms could include the following-
- Stored trouble code, and an illuminated warning light
- Additional codes may be present along with P0256
- Fuel consumption may increase
- Hard starting condition may be present
- In some cases, a no-start condition may be present
- Some applications may exhibit misfires, a general loss of power, or the engine may stumble or hesitate upon acceleration
- In some cases, excessive smoke may be visible from the tail pipe
NOTE: In extreme cases, catalytic converter and/or DPF (Diesel Particulate Filter) damage and/or failure may occur if the code is not resolved in a timely manner.
How do you troubleshoot code P0256 ?
SPECIAL NOTES: Since code P0256 – “Injection Pump Fuel Metering Control “B” Malfunction (Cam/Rotor/Injector)” can have many and varied causes and contributing factors, including clogged fuel filters and failures and malfunctions in the EVAP system, it is critically important to note the order in which additional codes (if any are present) are stored.
One or more of the codes that precede P0256 may have contributed to P0256 (if they have not actually caused the code), which means that these codes MUST be investigated and resolved before a diagnosis of P0256 is attempted. Failure to do this will almost certainly result in confusion, a misdiagnosis, wasted time, and the almost-certain unnecessary replacement of parts and components. Bear in mind that additional codes that follow P0256 will have set as the result of P0256 being set, and in most cases, these codes will not require any attention beyond clearing.
Also take note that since manufacturers do not always follow convention when it comes to labeling circuits, parts, and components, it is important to refer to the manual for the affected application for details on exactly which circuit is labeled “B”, since most applications also have a circuit “A” in the fuel control system. END OF SPECIAL NOTES.
Assuming that there are no additional codes present, or that all preceding codes have been resolved but P0256 persists, record all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
Refer to the manual to locate and identify all relevant components, as well as the function, routing, and color-coding of all associated wiring, including relevant fuses, fusible links, and relays. Bear in minds that even though fuses do not always need a reason to blow, a blown fuse could indicate a short circuit, so do NOT replace blown fuses until it is certain that there are no short circuits in the wiring.
Perform a thorough visual inspection of all associated wiring, and look for damaged, burnt, shorted, or corroded wiring and/or connectors. Make repairs, or replace wiring as required.
If no visible damage to wiring and/or connectors is found, prepare to perform ground, resistance, continuity, and reference voltage tests on all associated wiring, but be sure to disconnect the system from the PCM to avoid damage to the controller during resistance/continuity tests.
Note that while testing procedures vary somewhat between manufacturers and applications, the first test in this step would be to check that the correct voltage reaches the Fuel Rack Position sensor, which is often integrated into the Fuel Control Actuator. Thus, make sure that the correct wire is located in the connector, and test the voltage between the connector and a suitable ground with the key “ON”, but the engine not running. Depending on the application, this voltage will be either 5V, or 12V.
If this voltage does not check out as per the value specified for the application, test the output for this wire directly on the PCM connector as per the instructions provided in the manual. If the PCM does not deliver the correct voltage at this terminal, replace the PCM. If however, the PCM does deliver the correct voltage, inspect the wire between the PCM and the sensor connector. Repair or replace wiring as required to ensure that the electrical value falls within the range specified by the manufacturer.
If the power circuit checks out but the code persists, test the signal wire for resistance and continuity. With the key “ON” and the engine not running, the voltage on this circuit should read 5V as tested between the sensor connector and the PCM connector. If the reading is not 5V, inspect the wire, and repair or replace it to ensure that the sensor delivers a 5V (or very close to a 5V) signal to the PCM.
If the signal circuit is free of damage and its resistance agrees with the value stated in the manual, suspect either a faulty Fuel Rack Position sensor, or a faulty Fuel Control Actuator. Refer to the manual for the correct procedure to follow to test the actuator/sensor, but be sure to follow the instructions exactly to obtain the most accurate test results. Replace the sensor/actuator if its resistance/continuity does not fall within the range specified by the manufacturer.
Bear in mind that sensor/actuator failures are fairly common, so always use OEM replacements to ensure optimum performance and a reasonable service life.
Clear all codes after repairs are complete, and operate the vehicle for at least one complete drive cycle before rescanning the system to see if the code returns. If the code does not return after having completed several drive cycles, the repair can be seen as successful.
If however, the code does return, it is likely that there is an intermittent fault present. If this is suspected, repeat all electrical tests until the fault is found and repaired, or refer the vehicle to the dealer or other competent repair facility, since finding and repairing faults of this nature can be extremely challenging and time consuming.