P0400 — Exhaust Gas Recirculation Flow Malfunction
Moderate severityWhat it means
P0400 is stored when the engine control unit sees a fault involving the exhaust gas recirculation (EGR) system. The EGR system routes a little exhaust gas back into the intake to lower combustion temperature and NOx emissions. The control unit measured a flow that does not match what it commanded, so the system is not moving air or vapour as expected.
Common symptoms
- Check engine light with no strong driveability change
- Pinging or knock under load
- Rough idle or stalling
- Failed emissions test
Likely causes
- Blocked passage, stuck valve or solenoid
- Cracked or disconnected hose
- Carbon-clogged EGR valve or passages
- Faulty EGR valve, solenoid or position sensor
What to check first
- Inspect the related hoses, valve and solenoid for blockage or leaks
- Remove and inspect the EGR valve and passages for carbon
- Command the valve with a scan tool and watch the engine response
Can you keep driving?
Often drivable, but pinging under load can damage the engine over time — avoid heavy loads until repaired.
Scan, log and diagnose with MechanicAI
Plug a Bluetooth OBD2 adapter into your car and let MechanicAI turn raw data into answers:
- Scan, read and clear fault codes in seconds
- Inspect every sensor with live data
- Create instant logs and automatic trip records
- And above all — analyze it all with AI for the most likely cause and clear next steps.
Related codes
All information, explanations and guidance on this page are general preliminary information only. They are estimates and may be entirely wrong, incomplete or inaccurate, or may not apply to your vehicle at all. Never take any action based on this content alone — do not decide to keep driving, clear codes, replace parts, repair or otherwise work on your vehicle based only on what you read here. In every case, and for any final decision, you must consult qualified technical personnel (a professional mechanic, an authorized service centre or an engineer) and obtain their approval before acting. MechanicAI accepts no liability or responsibility whatsoever for any damage, fault, injury, loss or outcome that may arise — directly or indirectly — from using or relying on this information, for any reason and under any circumstances. By using this page you acknowledge and accept these terms.