P0400
Exhaust Gas Recirculation Flow Malfunction
A general fault has been detected in the exhaust gas recirculation (EGR) system. EGR routes a portion of exhaust back into the intake to reduce combustion temperature and NOx emissions. Most EGR codes today are caused by carbon buildup blocking the valve.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- EGR valve clogged with carbonCommon
- Intake EGR passages blockedPossible
- DPFE / EGR position sensor failedLess likely
Parts involved
- EGR Valve
- EGR Cooler
- Dpfe Sensor
P0400 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.