P0400 Exhaust gas recirculation (EGR) A flow malfunction
What P0400 means
The EGR valve recirculates a controlled amount of exhaust gas into the intake to reduce NOx emissions. P0400 is a general flow fault: the engine computer saw EGR flow that did not match what it commanded, either too little or too much. Carbon build-up is the usual cause.
P0400 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- Medium
- Typical repair cost
- 80–600 €
- Workshop time
- 1–5 h
- Repair difficulty
- Medium
- DIY possible
- Yes
- Check engine light
- Yes
- Affects emissions test
- Yes
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Check engine light on
- Rough idle or stalling if the valve sticks open
- Knocking under load if flow is too low
- Reduced power on diesels
- Technical inspection failure for NOx
Common causes
- Carbon deposits in the EGR valve or passages
- EGR valve or actuator sticking
- Faulty EGR position sensor
- Leaking vacuum hose or faulty vacuum solenoid
- Damaged EGR valve wiring
Can I keep driving with P0400?
You can drive, but have it repaired within one or two weeks. If the engine idles roughly or stalls, the valve may be stuck open and should be checked sooner.
What to check first
- Check freeze-frame data and look for related EGR position or MAP sensor codes
- Command the EGR valve with a scan tool and compare commanded and actual position
- Remove and clean the valve and passages if carbon is present
- Check vacuum hoses and wiring, and replace the valve if it still does not follow commands
Related codes
- P0401 Exhaust gas recirculation (EGR) A flow insufficient
- P0402 EGR flow excessive detected
- P0403 EGR A control circuit/open
- P0404 EGR A control circuit range/performance
- P0405 EGR sensor A circuit low
- P0406 EGR sensor A circuit high
- P0410 Secondary air injection system A
- P0411 Secondary air injection system incorrect flow detected
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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