Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.

P0400: Exhaust gas recirculation flow malfunction

The engine computer has detected that exhaust gas recirculation (EGR) flow is not behaving as expected.

What P0400 means

The engine computer has detected that exhaust gas recirculation (EGR) flow is not behaving as expected. P0400 is a broad flow fault: it does not by itself state whether flow is too little or too great, or whether the valve is the failed component.

Can I still drive?

With a steady light and normal running, restrict use to gentle driving while arranging prompt diagnosis. Stop and recover the vehicle if it stalls, runs very roughly, loses substantial power, knocks heavily, overheats or displays a flashing or red warning. Poor EGR control can upset combustion and emissions even when the car feels normal.

Symptoms

  • Engine-management light, sometimes with no drivability change.
  • Rough or fluctuating idle.
  • Hesitation, surging or stalling.
  • Reduced power or economy.
  • Pinging or knocking under load when commanded flow is absent.

Common causes

  • Carbon restricting the EGR valve, cooler, pipework or intake passage.
  • EGR valve sticking open or closed.
  • Split, blocked, disconnected or misrouted vacuum hose.
  • Faulty control solenoid or electrical actuator.
  • Incorrect feedback from a pressure, temperature, MAP, MAF or position sensor.
  • Wiring, connector, earth, exhaust-leak or—rarely—module fault.

Safe first checks

  • Save all codes and freeze-frame data. On a cold, switched-off engine, inspect accessible EGR plugs, wiring and vacuum hoses for loose connections, chafing, corrosion or splits. Look for soot marks around pipe joints and note companion airflow, pressure-sensor, circuit or misfire codes. Do not condemn the valve from P0400 alone; proving commanded movement and measured flow often needs vehicle-specific data and a bidirectional scan tool.

Typical UK fix cost

Allow £90–£180 for diagnosis. Cleaning an accessible valve or passage may total £150–£350; a valve, solenoid, sensor or wiring repair is commonly £250–£900. An EGR valve/cooler assembly or awkward removal can reach £700–£1,500+.

Related models

This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.

Related fault codes

Sources

A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.