Wiring or connector fault in the fuel metering control circuit (open circuit, short, corrosion or chafing); Faulty fuel metering valve, spill valve or pump control solenoid; Low fuel supply pressure from a blocked fuel filter, air ingress or weak lift pump; Worn or faulty injection pump internals (cam, rotor or position feedback sensor); Poor power or earth supply to the pump or control unit, such as a weak battery or bad earth; Contaminated or poor quality fuel, including water in the fuel; Fault inside the engine control unit or pump driver module.

P0256

Injection Pump Fuel Metering Control B (Cam/Rotor/Injector).

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
2: AreaFuel and air metering (injector circuit)
56: FaultThe specific fault in that area

What it means

P0256 is the generic SAE J2012 code for a fault in Injection Pump Fuel Metering Control 'B' circuit or system, relating to the cam, rotor or injector side of the pump on diesel engines. The engine control unit has seen that the commanded fuel metering does not match the feedback, or that the control circuit is open, shorted or out of range. The exact component behind 'B' and the strategy used to set the code depend on the make, model and pump design.

Symptoms

Engine management light on
Hesitation or poor acceleration
Reduced power or limp mode
Rough idle or uneven running
Hard starting or long cranking
Engine stalling or surging

Possible causes

Wiring or connector fault in the fuel metering control circuit (open circuit, short, corrosion or chafing)likely
Faulty fuel metering valve, spill valve or pump control solenoidlikely
Low fuel supply pressure from a blocked fuel filter, air ingress or weak lift pumppossible
Worn or faulty injection pump internals (cam, rotor or position feedback sensor)possible
Poor power or earth supply to the pump or control unit, such as a weak battery or bad earthpossible
Contaminated or poor quality fuel, including water in the fuelpossible
Fault inside the engine control unit or pump driver modulerare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including engine speed, load and temperature, to see when the fault occurred.
Check for related codes such as fuel rail pressure, pump position sensor or supply voltage faults, as these point towards the root cause.
Inspect the wiring and connectors to the injection pump and metering valve for damage, corrosion, water ingress and loose pins.
Check battery condition, charging voltage and the relevant earth connections.
Inspect the fuel system for leaks, air in the fuel, a blocked filter and fuel contamination, and check the fuel supply pressure against the manufacturer's specification.
Using a scan tool, view live data for commanded versus actual metering or pump position values and watch for deviation.
Test the metering valve or solenoid resistance and the control signal at the connector, following the manufacturer's procedure for that engine.
Only if everything else checks out, assess the pump and control unit following the manufacturer's procedure, as this depends on the pump type.

How to fix

Repair or replace damaged wiring and clean or replace faulty connectors.
Replace the fuel metering valve or solenoid if it fails testing.
Replace the fuel filter, repair air leaks and restore correct fuel supply pressure, and drain and replace contaminated fuel.
Repair or replace the injection pump if internal wear or a faulty feedback sensor is confirmed.
Correct battery, charging or earth faults, then clear the code and road test to confirm it does not return.
Update or replace the control unit software or hardware only where the manufacturer lists it as a known fix.

Can I keep driving?

Medium severity

The car may be driveable in limp mode, but the fault can worsen and cause stalling or loss of power, which is a safety risk in traffic. Get it checked soon and avoid long journeys or heavy loads until it is fixed.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.