P0400
Exhaust Gas Recirculation Flow
- Severity
- Medium
- Driving
- Drivable — book it in
The exhaust gas recirculation system is not behaving as the control module expects. EGR deliberately feeds a small amount of inert exhaust gas back into the intake to lower combustion temperature and cut nitrogen oxide emissions. This is the general code for that system being wrong without the module committing to whether flow is too high or too low - a more specific code alongside it will usually say which.
Possible causes
- CommonCarbon build-up in the EGR valve or its passages
- CommonEGR valve stuck open or stuck shut
- PossibleFaulty EGR position or differential pressure sensor
- PossibleVacuum leak or collapsed vacuum hose on a vacuum-operated valve
- PossibleFaulty EGR control solenoid
- PossibleWiring fault between the module and the EGR actuator
What you might notice
- Check engine light on
- Failed emissions test
- Rough idle
- Pinging or knocking under load
- Often no symptom the driver notices
What to check, in order
- Read all stored codes before starting. This code on its own is vague, but P0401 (too little flow) or P0402 (too much) alongside it points straight at the problem.
- Inspect the EGR valve and its mounting passage for carbon. Carbon is the cause in the large majority of EGR faults on a high-mileage engine.
- On a vacuum-operated valve, check the vacuum hose is intact and that vacuum actually reaches the valve when the module commands it.
- Watch the commanded EGR position against the actual position on the live data screen. A large persistent gap means the valve is not moving as instructed.
- Check the wiring and connector at the valve for corrosion or heat damage. The valve sits in a hot part of the engine bay and its connector suffers for it.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Auxiliary emission controls
P0400P0401P0402P0403P0404P0405P0406P0407P0408P0409P0410P0411P0412P0413P0414P0415P0416P0417P0418P0419P0420P0421P0422P0423P0424P0425P0426P0427P0428P0429P0430P0431P0432P0433P0434P0435P0436P0437P0438P0439P0440P0441P0442P0443P0444P0445P0446P0447P0448P0449P0450P0451P0452P0453P0454P0455P0456P0457P0458P0459P0460P0461P0462P0463P0464P0465P0466P0467P0468P0469P0470P0471P0472P0473P0474P0475P0476P0477P0478P0479P0480P0481P0482P0483P0484P0485P0486P0487P0488P0489P0490P0491P0492P0493P0494P0495P0496P0497P0498P0499