P0401 means the exhaust gas recirculation system is not flowing as much as the engine computer expects. It is nearly always carbon: the EGR valve, or the passages behind it, have clogged up. Diagnose soon and drive gently. Indicative UK repair cost is £80 to £450.
EGR works by feeding a measured amount of inert exhaust gas back into the inlet, which cools combustion and cuts oxides of nitrogen. The gas it is feeding back is dirty and the passages it travels through are narrow, so over tens of thousands of miles they coke up. A life of short journeys and stop-start traffic accelerates it, because the deposits never get hot enough for long enough to be scoured away.
Can I drive with P0401?
Diagnose soon, drive gently. Nothing dramatic happens with a blocked EGR. You might notice a slightly rougher idle, a bit more smoke under load on a diesel, or honestly nothing at all. The reason not to leave it indefinitely is indirect but real on a diesel: an EGR that is not working properly usually means more soot leaving the engine, more soot fills the particulate filter faster, and the filter is a far bigger bill than the valve.
What P0401 means for your MOT
The EGR valve is one of the very few components the MOT names directly, so this section is more concrete than it is for most codes.
- The valve as a part. Section 8.2.1.1 for petrol and 8.2.2.1 for diesel both instruct the tester to inspect visible, identifiable emissions components, and both list exhaust gas recirculation valves explicitly. A valve that is missing, obviously modified or obviously defective is a major defect on any vehicle first used on or after 1 September 2002.
- Which means a blanking plate fails. This is worth stating plainly, because EGR blanking kits are widely sold and widely fitted in the UK and plenty of owners have no idea where they stand. A blanked or deleted EGR is modified emissions equipment, and a tester who sees it should reject the car. A remap that hides the warning light does not change what is bolted to the engine.
- The lamp. Major defect under 8.2.1.2 for petrol first used on or after 1 July 2003 and 8.2.2.2 for diesel first used on or after 1 July 2008, per the DVSA MOT inspection manual.
What usually causes it
- A clogged or sticking EGR valve. The headline cause. Carbon builds up around the valve seat and the pintle until it cannot open its full travel, or cannot move smoothly enough to meet the flow the computer is asking for.
- Blocked EGR passages. Worth keeping separate from the valve, because this is the version where fitting a new valve changes absolutely nothing. The ports in the manifold, and the EGR cooler where one is fitted, can be close to solid, and clearing them properly means taking parts off the engine rather than spraying cleaner at them.
- A faulty EGR position or pressure sensor. The flow may be perfectly adequate and the measurement of it wrong. Cheaper than the alternatives and worth eliminating before anything is dismantled.
What it costs to fix in the UK
£80 to £450. Cleaning a valve in situ, on an engine where it can actually be reached, is the bottom. A replacement valve is the middle. The top of the range is an engine where the inlet manifold has to come off to get at the passages, and a blocked EGR cooler on top of that can take it further still.
Worth asking any garage whether their quote covers cleaning the passages or only swapping the valve, because those are different jobs at very different prices. What a car diagnostic costs in the UK covers what you should expect to pay just to have it looked at.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Find out whether anything has been blanked or remapped. This is the first question on this code rather than the last, because it changes the diagnosis, the repair and the legal position all at once. If you bought the car used, you may not know, and it is worth looking.
- Take the valve off and look at it. No amount of live data beats seeing whether the pintle is coated and whether it moves freely by hand. On many engines this is a two-bolt job.
- Compare commanded EGR position with actual. A valve being told to open 40 per cent and reporting 5 is sticking. One that reports what it was asked for while the flow fault persists moves your attention to the passages.
- Check the passages before you buy a valve. Shine a torch into the port once the valve is off. If you cannot see daylight through it, a new valve is money thrown at the wrong half of the problem.
Related codes
- P2002, diesel particulate filter efficiency low. The code that often follows this one on a diesel, for the soot reason set out above, and the expensive consequence of leaving it.
- P0299, turbocharger underboost. Shares the carbon problem. A heavily coked inlet affects both breathing and boost, and the two codes keep each other company on high-mileage diesels.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.