P0400
PowertrainEGR Flow Malfunction
The EGR system routes a measured amount of exhaust gas back into the intake to lower combustion temperatures and cut nitrogen oxide emissions. P0400 means the ECU has worked out that the gas isn't flowing the way it should, usually too little of it getting through. On most cars this comes down to a valve choked with carbon, and you'll feel it as rough running and a warning light long before anything dramatic happens.
ⓘ Information only. This page provides general educational information about fault code P0400. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0400 mean?
P0400 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: EGR Flow Malfunction.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, often the only thing you'd actually notice
- • Lumpy or unstable idle once the engine has warmed up
- • A flat spot or hesitation when you ease onto the throttle
- • Fuel economy creeping down over a few tankfuls
- • Pinking or knocking under load on some petrol engines because combustion temps have climbed
- • Stalling or hard starting if the valve has stuck wide open
Possible causes
Causes commonly associated with P0400, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Carbon caked up inside the EGR valve or the intake passages. This is far and away the usual story on diesels past 80,000 miles, the VW 1.9 and 2.0 TDI and the PSA 1.6 HDi are classic offenders for sooting up
- 2. EGR valve sticking and no longer opening or closing cleanly, sometimes seized solid
- 3. Blocked EGR tube or clogged passages in the intake manifold restricting flow even when the valve is fine
- 4. Vacuum leaks, split hoses or a disconnected pipe on older vacuum-operated systems
- 5. A duff EGR position sensor or differential pressure sensor feeding the ECU bad readings
- 6. Corroded connectors or chafed wiring in the EGR circuit
- 7. Failed EGR control solenoid or vacuum pump on certain diesels
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the freeze-frame data stored with the code and note any other codes alongside it. A P0401 or a fuel trim code points you in a very different direction to a wiring fault
- 2. Have a good look over the vacuum hoses, connectors and wiring for cracks, corrosion or anything hanging loose. Cheap to check, cheap to fix if that's all it is
- 3. With a bi-directional scan tool, command the valve open at idle. A working valve will make the engine stumble or nearly stall, no change at all means little or no flow is getting through
- 4. Pull the valve off and inspect it and the intake ports for carbon. On a sooty diesel this is often where the whole job ends, a clean and refit
- 5. Test the valve itself: a hand vacuum pump on vacuum-actuated types, or a multimeter for resistance and signal on electronic ones
- 6. Clear the code and drive it under the same conditions that triggered it to confirm the fault is actually gone and not just reset
Common questions about P0400
What am I risking if I just leave it and keep driving? +
Short term you're mostly looking at rough running, worse mpg and a warning light. The bigger worry is the engine running hotter in the cylinders without that exhaust gas cooling things down, which on a petrol can bring on pinking under load and over time stress the engine. On a diesel a stuck-open valve can also dump soot through the intake and clog things further. It'll get you home, but don't ignore it for months.
How quickly do I need to sort this out? +
It's not a pull-over-now emergency in most cases, so you've got time to book it in rather than panic. That said, if the idle is hunting badly, it's stalling, or you can hear knocking under acceleration, get it looked at within a week or so before the carbon problem snowballs or the warm running damages something. A cheap clean now beats a seized valve and a manifold full of soot later.
Is it the EGR valve itself that's gone, or could it be the wiring? +
Most of the time it's the valve or the passages choked with carbon, particularly on higher-mileage diesels. Wiring and connector faults do happen, usually showing up as a position-sensor code or an intermittent fault that comes and goes, so it's always worth wiggling the connector and checking for green corrosion before you condemn the valve. If the contacts are clean and the valve still won't respond when commanded, the valve or its solenoid is your culprit.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →