Fuel and air

P0251 — Injection Pump Fuel Metering Control A Malfunction

Severity: medium Drive with care, to a shop

What P0251 means

P0251 applies to diesel engines with a mechanically or electronically controlled injection pump. The ECM commands the amount of fuel delivered through a metering actuator (a rack or metering valve) and checks the result with a position sensor. If the actual position doesn't match the command, or the circuit is open, shorted or has no signal, the code is stored and the check engine light comes on.

Symptoms

  • Check engine light on
  • Reduced power and sluggish acceleration
  • Rough idle or misfire
  • Hard starting or stalling under load
  • Worse fuel economy
  • White smoke from the exhaust
  • No-start

Possible causes

  1. Faulty fuel metering actuator or failing injection pump
  2. Damaged wiring: open, short or corroded connectors
  3. Faulty fuel rack position sensor
  4. Air in the diesel fuel system
  5. Contaminated or poor-quality fuel
  6. Injection pump timing out of specification
  7. Internal ECM fault (rare)

How to diagnose

  1. Read all codes and freeze-frame data and look for other fuel system codes
  2. Inspect the pump harness and connectors for chafing, oil, corrosion and loose pins
  3. Check supply voltage and ground at the metering actuator connector
  4. Measure actuator and position sensor resistance and compare with specs
  5. Check fuel pressure, filter condition, fuel quality and air in the lines
  6. Compare cylinder balance and watch metering position data live
  7. Verify injection pump timing marks
  8. Suspect the ECM only after that, after checking its power and output signal

Common fixes

  • Repair the wiring and connectors in the metering circuit
  • Replace the rack position sensor or metering actuator
  • Replace the fuel filter, bleed air and clean out contaminated fuel
  • Repair or replace the injection pump
  • Reprogram or replace the ECM if confirmed faulty

Questions and answers

What does P0251 mean?

The diesel ECM found a fault in the injection pump's fuel metering control A circuit. The cause may be wiring, a sensor, the actuator or the pump itself.

Can I drive with a P0251 code?

Only for short distances and with caution. The engine may lose power or stall, and if the metering actuator fails it may not start at all.

What should I check first for P0251?

Start with the pump wiring and connector, then the actuator's power and ground, then the fuel filter and any air in the system. If those are fine, move on to the sensor and the pump.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.