P0400 Exhaust Gas Recirculation Flow Malfunction

P0400 points to an exhaust gas recirculation flow malfunction. The EGR system did not show the flow behavior expected by the monitor.

Code details

Generic OBD-II · P0

System: Emissions
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the EGR valve based only on the label; low or abnormal flow can come from passages, controls, sensors, or wiring.

Symptoms of P0400

Check-engine light, often with no obvious change in how the car drives.
A failed emissions test or incomplete readiness monitors.
Sometimes a faint fuel-vapour smell (on EVAP codes).

Common causes of P0400

A worn catalytic converter or an oxygen/air-fuel sensor reading out of range.
EVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve.
Exhaust leaks, fuel-trim problems, or a fault in a related monitor.

Useful checks before replacing parts

Check EGR valve operation, vacuum or electrical control, hoses, passages, and feedback sensor data.
Review freeze-frame conditions and command EGR flow if the scan tool supports active testing.
Inspect for carbon blockage, stuck valves, broken vacuum lines, and wiring faults before replacing components.

Related OBD-II codes

These nearby emissions entries can help compare wording and pin down the fault area.