EGR Fault Codes UK — What P0400, P0401, P0402, P0403 and P0411 Mean

Published 9 Jul 2026 • EGR faults • UK repair costs

Last reviewed 9 July 2026 The AI-Diagnostics-Pro Team

This guide is reviewed against current UK EGR-diagnosis patterns, indicative repair-cost context, and DVSA MOT guidance where a live EGR fault affects test outcomes.

Editorial policy DVSA MOT inspection manual

The Exhaust Gas Recirculation (EGR) system recycles a portion of exhaust gas back into the intake to lower combustion temperatures and reduce NOx emissions. When it faults, the engine management light comes on and you may notice rough running, power loss, or increased smoke — especially on UK diesel cars where short trips and urban driving accelerate carbon buildup inside the EGR passages.

What each EGR fault code means

P0400 — EGR Flow Malfunction. The ECU has detected that exhaust gas is not flowing through the EGR system at the expected rate. This is the broadest EGR code and usually points to a blockage, a stuck valve, or a control fault rather than a single failed part. On UK diesels it is extremely common above 60,000 miles.

P0401 — EGR Flow Insufficient. Too little exhaust gas is recirculating. The most frequent UK cause is carbon buildup in the EGR passages or on the valve pintle, not a failed valve itself. The valve may still move correctly while the flow path around it is restricted.

P0402 — EGR Flow Excessive. Too much exhaust gas is entering the intake. This usually means the EGR valve is stuck open, the vacuum actuator is holding it open, or the control solenoid is commanding excessive flow. You will often notice rough idle and increased smoke.

P0403 — EGR Circuit Malfunction. An electrical fault in the EGR control circuit — open circuit, short circuit, or poor connector contact. This code is about wiring and electronics rather than carbon or flow restriction.

P0411 — Secondary Air Injection Incorrect Flow. Although not strictly an EGR code, P0411 sits in the same emissions-flow family. It means the secondary air injection system is not delivering the expected airflow during cold start. On UK petrol cars this commonly comes from a faulty air pump, stuck switching valve, or water-damaged hoses.

Common symptoms on UK cars

Engine management light on steady or flashing
Rough idle, especially when cold or after short trips
Loss of power under acceleration
Increased black smoke from the exhaust (diesel)
Fuel economy deterioration
Limp mode on some vehicles when the fault is severe

What to check first

Carbon buildup. On UK diesel cars this is the number one cause. Remove the EGR valve and inspect the pintle and passages for heavy soot deposits. Cleaning often resolves P0400 and P0401 without replacing parts.
Vacuum lines and actuator. For vacuum-operated EGR valves, check that the vacuum pipe is intact, the solenoid responds to command, and the actuator moves the valve through its full range.
Wiring and connectors. For P0403, inspect the EGR connector for corrosion, check wiring continuity, and confirm power and ground at the solenoid.
Related faults. Look for DPF, boost, or intake codes that may be contributing to low or high EGR flow readings. Fixing a blocked DPF sometimes resolves an apparent EGR fault.
Live data. If your scan tool supports it, compare commanded versus actual EGR position or flow. A large discrepancy points to mechanical restriction or a control-side fault.

Typical UK repair costs

Likely fix	Typical UK cost	Notes
EGR valve cleaning	£80–£200	Most common fix for P0400/P0401 on UK diesels. Labour-heavy because of access.
EGR valve replacement	£150–£500	Only needed if the valve is genuinely stuck or the solenoid has failed. OEM parts cost more.
Vacuum solenoid replacement	£60–£150	Common on vacuum-controlled EGR systems. Quick job on most cars.
EGR cooler replacement	£250–£600	Needed when the cooler leaks internally, mixing coolant with exhaust gas.
Intake manifold removal and cleaning	£200–£400	Required when carbon has built up inside the manifold runners, not just the EGR valve.
Secondary air pump (P0411)	£150–£400	Petrol only. Pump, valve, or hose replacement depending on the failed component.

MOT risk is real. An active EGR fault code will usually cause an MOT emissions failure on diesel cars. The engine management light being on alone can also contribute to a fail. Fix the fault before your test date.

When to see a garage versus DIY

Cleaning an EGR valve is a reasonable DIY job if you are comfortable removing the valve and scraping carbon deposits. However, on many UK cars the EGR valve is buried under the intake manifold, which means several hours of labour and specialist tools. If the code is P0403 (electrical), a garage can usually diagnose the wiring fault quickly with a multimeter. For P0411, the secondary air pump and valves are often accessible, but fault-finding the airflow path still benefits from professional scan-tool data.

Prevention tip. Regular motorway runs at sustained speed help keep EGR passages clear by raising exhaust temperatures and burning off carbon. UK drivers who only do short urban trips are far more likely to develop EGR blockage.

Related guides

P0400 P0401 P0402 P0403 P0411

→ DPF fault codes UK — P2002, P2003, P2459 guide → Will an engine management light fail MOT?→ How to pass your MOT first time → Flashing check engine light — what it means and what to do

Not sure which EGR fault you have?

Enter your fault code and vehicle details for a full AI diagnostic report — repair costs, MOT risk, and step-by-step fix guidance for your exact car.

Get My Report — £1.59

AI-Diagnostics-Pro provides AI-generated vehicle diagnostic information for informational and educational purposes only. It is not a substitute for a professional vehicle inspection or diagnosis by a qualified automotive technician. Always verify safety-critical issues with a qualified automotive professional before carrying out repairs or continuing to drive your vehicle.