Faulty fuel metering control solenoid or actuator on the injection pump; Damaged, chafed, or corroded wiring in the 'B' control circuit; Loose or contaminated electrical connector at the pump or ECM; Internal injection pump mechanical wear affecting the cam ring or rotor; Failed ECM driver circuit for the fuel metering control output; Poor ground connection for the injection pump or engine block

DTC codes / P02xx · Fuel and air metering (injector circuits) / P0256

P0256Injection Pump Fuel Metering Control 'B' (Cam/Rotor/Injector)

P0256 is a generic OBD-II code that indicates the engine control module (ECM) has detected an abnormal electrical condition in the fuel metering control circuit for injection pump output 'B', which is often associated with the cam ring, rotor, or injector circuit on diesel injection pumps. The ECM monitors the commanded current or voltage on this circuit and sets the code when the actual signal falls outside the expected range for a calibrated period.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaFuel and air metering (injector circuits)

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Rough idle or misfire, especially when the engine is cold
Reduced engine power or limp mode during acceleration
Hard starting or extended cranking before the engine fires
Increased fuel consumption or black smoke from the exhaust
Intermittent stalling or hesitation under load

Common causes, most likely first

Faulty fuel metering control solenoid or actuator on the injection pump
Damaged, chafed, or corroded wiring in the 'B' control circuit
Loose or contaminated electrical connector at the pump or ECM
Internal injection pump mechanical wear affecting the cam ring or rotor
Failed ECM driver circuit for the fuel metering control output
Poor ground connection for the injection pump or engine block

How to diagnose and fix it

Scan all modules and record freeze-frame data to confirm when the code sets
Inspect the injection pump harness and connectors for damage, corrosion, or loose pins
Measure resistance and voltage on the 'B' control circuit and compare to manufacturer specifications
Test the fuel metering control solenoid for proper resistance and function
Check fuel pressure and volume to rule out supply issues that can mimic circuit faults
Perform a wiggle test on the harness while monitoring live data to catch intermittent faults
If all electrical checks pass, follow the manufacturer's diagnostic flow for injection pump replacement or ECM repair

What the P0256 code often hides

A weak or failing fuel pump relay or fuse can cause voltage drops that set P0256 even though the metering solenoid itself is good.
On many diesel engines, air in the fuel system from a leaking filter housing or supply line can trigger this code by causing erratic metering control feedback.
Technicians sometimes replace the injection pump only to find the real problem is a chafed wire behind the engine block that shorts intermittently when hot.
P0256 can be set by a low battery or charging system fault because the ECM may misinterpret low system voltage as a circuit failure.
Always check for technical service bulletins (TSBs) for the specific vehicle, as some manufacturers have updated harness routing or connector designs to prevent this code.

Can I keep driving with P0256?

You can usually drive the vehicle to a repair shop if the engine runs smoothly and the check engine light is steady, but avoid heavy acceleration or towing. If the engine misfires, stalls, or enters limp mode, stop driving and have it towed to prevent damage to the injection pump or catalytic converter.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.