EGR Fault Codes: P0400 to P0409 Decoded

OBD fault codes in the P0400 series relate to the exhaust gas recirculation system, and they're among the most frequently encountered codes on modern diesel vehicles. While they all relate to the EGR system, they describe different aspects of its operation and point to different potential causes. Here's what each one means.

P0400 is a general EGR flow malfunction code — it indicates that the EGR system is not functioning within expected parameters, but doesn't specify whether flow is excessive or insufficient. This is often the first code to appear when an EGR valve is developing carbon build-up and beginning to stick. It may be accompanied by P0401 (insufficient EGR flow detected) or P0402 (excessive EGR flow detected), which provide more specific direction.

P0401, insufficient EGR flow, is typically caused by a stuck-closed or heavily carbon-coated EGR valve that isn't opening fully when commanded. The exhaust gas isn't being recirculated as the ECU expects, which affects emissions calculations and engine management. On a diesel, this is very commonly caused by carbon build-up and is a strong indicator that an EGR clean is needed.

P0402, excessive EGR flow, indicates the opposite — more exhaust gas is entering the intake than expected. This often points to a stuck-open EGR valve, where carbon deposits are holding the valve in a partially or fully open position. At idle, this is particularly problematic as it introduces exhaust gas when the intake should be receiving clean air only.

P0403 indicates an EGR circuit malfunction — an electrical fault rather than a flow issue. This could be a faulty EGR actuator, a wiring issue, or a failed position sensor. It requires a more detailed electrical diagnosis rather than simply a physical clean. P0404 (EGR circuit range/performance) is similar and relates to the valve not moving to the position commanded by the ECU.

P0405 and P0406 relate to the EGR position sensor circuit — low and high voltage readings respectively. These are electrical faults in the sensor that monitors valve position. P0407 and P0408 are similar for the secondary EGR sensor where fitted.

When these codes appear, we always recommend a full diagnostic before deciding on a course of action. Sometimes a code points directly to a clean or replacement; other times it indicates a sensor or wiring fault that a clean won't resolve. Knowing the difference saves time and money.