P0400 indicates exhaust gas recirculation flow malfunction. The exact repair depends on whether the underlying issue is a sensor fault, wiring problem, mechanical wear, or a related system problem on the vehicle being diagnosed.
This guide is reviewed against current UK fault-code usage patterns, MOT guidance, and indicative repair-cost context. It is educational guidance for P0400, not a substitute for live data, workshop testing, or a final quote.
This page explains the code in plain English and gives indicative UK repair and MOT context. It does not confirm the exact failed part. Vehicle age, engine, related codes, live data, and workshop inspection can all change the final diagnosis and quote.
P0400: EGR system not flowing correctly — often carbon buildup or a stuck valve. Check EGR-specific tests first (£50-£500). MOT fail risk. When this fault is detected, the ECU (Engine Control Unit) stores the code and usually illuminates the check engine light on the dashboard.
In practice, P0400 should be treated as a egr and emissions flow problem first, not as proof that one named part has failed. The value of the code is in narrowing the system, checking related data, and avoiding the usual misdiagnosis before parts are ordered.
These are the most common symptoms UK drivers report when this code is active:
Note that some vehicles show no drivability symptoms at all — the check engine light may be the only sign. This is why scanning the fault code is essential rather than guessing at the cause.
These are typical causes to check first:
It is important to diagnose the root cause before replacing parts. Many unnecessary and expensive repairs happen because the most expensive component is replaced first without proper testing. Always work through the diagnostic steps systematically.
Work through these steps in order to avoid unnecessary parts replacement:
After any repair, clear the fault code with an OBD-II scanner and carry out a test drive. If the code returns, continue to the next diagnostic step. For a personalised repair guide specific to your vehicle's make, model, year and engine, use our AI Diagnostic Report below.
These models are frequently seen with this fault code in UK garages:
This code can appear on OBD-II compatible vehicles. OBD-II is standard on most UK petrol cars from 2001 and diesel cars from 2004, with many later-1990s vehicles also compatible. The models listed above are examples where owners and garages often encounter this fault, but they are not an exhaustive or ranked list.
Difficulty: Medium — EGR cleaning is DIY but messy. Cooler work needs a professional.
UK repair cost estimate: £50–£500 including parts and labour at average UK workshop rates (inc. VAT). Independent garages are typically 30–50% cheaper than main dealers. Always get at least two quotes.
Prevention: Use quality diesel, take regular longer runs to clear carbon, consider EGR cleaning every 40,000 miles.
P0400 is the broad EGR flow fault, so it often sits earlier in the diagnostic tree than P0401 or P0402. On UK diesels it commonly reflects carbon buildup, vacuum/control faults, or flow readings that do not match expectation.
Replacing the EGR valve as a complete unit without proving whether the problem is flow, control, or blockage is the usual waste.
Fault code P0400 is likely to cause an MOT failure. A stored code does not automatically fail an MOT, but if it keeps the malfunction indicator lamp (MIL / check engine light) on where MIL inspection applies, or affects emissions, it can cause a failure.
Even if the light is not currently on, if the fault is intermittent it could reappear during the test. Use our AI Diagnostic Report to understand the MOT risk specific to your vehicle and get advice on whether it is safe to proceed to your test.
This guide is informed by the following authoritative UK sources:
Repair cost estimates are based on UK labour rates and parts pricing from AA, RAC and independent garage surveys, reviewed quarterly. All content is for educational purposes and is not a substitute for professional mechanical diagnosis.
Specific to your vehicle — year, make, model and engine.
Includes step-by-step repair guide, exact UK labour costs and MOT impact.
What does P0400 mean in plain English?
P0400 is an egr and emissions flow fault code. In practice, P0400 should be treated as a egr and emissions flow problem first, not as proof that one named part has failed. The code helps narrow the system, check related data, and avoid misdiagnosis before parts are ordered.
Will P0400 fail an MOT?
P0400 is likely to affect MOT outcome if the fault is active, keeps the warning light on, or causes emissions failure.
Can I keep driving with P0400?
Usually driveable but emissions and performance will be affected. Likely MOT failure.
How much does it usually cost to fix P0400 in the UK?
Typical UK repair costs for P0400 range from £50–£500, depending on the underlying fault, your exact vehicle, and local labour rates.
Use these supporting pages to compare likely repair paths, warning-light behaviour, MOT risk, and the questions to ask before approving work.
Compare P0400 with closely related faults, nearby codes in the same family, and adjacent module or sensor faults that often need to be ruled out during diagnosis.
Last updated: August 2026. AI-Diagnostics-Pro provides AI-generated information for educational purposes only. It is not a substitute for professional automotive advice. Always consult a qualified mechanic before carrying out vehicle repairs. Repair cost estimates are approximate and may vary by region and vehicle.