P0401: Exhaust Gas Recirculation "A" Flow Insufficient Detected
P0401 means Exhaust Gas Recirculation "A" Flow Insufficient Detected. Exhaust gas recirculation (EGR) sends a small amount of exhaust back into the intake to lower combustion temperatures and NOx emissions. This code means less EGR flow than expected was detected, which usually points to carbon-blocked passages or a valve that does not open.
Carbon is the usual enemy here. EGR passages slowly clog with soot, especially on engines that are mostly driven short distances.
Can I keep driving? Drivable with care. Diagnose soon: performance, economy or other components can be affected.
Common symptoms
- Check engine light on
- Pinging/knocking under load (insufficient flow)
- Rough idle or stalling (excessive flow)
- Failed emissions test
Possible causes
- Carbon build-up in the EGR valve or passages
- Faulty EGR valve
- Faulty EGR position or differential pressure sensor, or its hoses
- Vacuum supply problem on vacuum-operated valves
- Faulty EGR control solenoid
How to diagnose P0401
- Inspect the EGR valve and passages for carbon.
- Command the EGR valve on a scan tool and watch the position and engine response.
- Check sensor hoses and vacuum supply.
- Clean or replace the valve as needed.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.