P0400: Exhaust Gas Recirculation "A" Flow
The engine control unit has detected that EGR flow is not as expected.
Normal risk
Usually drivable, but emissions rise and running may suffer. Have it checked soon.
Short answer · Can I keep driving with P0400?
Usually yes. The engine may knock, hesitate or run unevenly. Have the EGR system checked soon.
What does P0400 mean?
Code P0400 is a general fault in exhaust gas recirculation flow: the measured effect of EGR does not match what was commanded. Carbon deposits, a sticking valve, blocked passages or a faulty sensor are common causes.
What the car does when the code is set
The check engine light comes on. Most cars drive normally, though some petrol engines knock under load and some diesels feel less smooth. Nitrogen oxide emissions are higher.
How the system works
Exhaust gas recirculation feeds a controlled amount of exhaust gas back into the intake. The inert exhaust gas lowers the peak combustion temperature, which reduces nitrogen oxide (NOx) emissions and, on petrol engines, helps prevent knocking. An electrically or vacuum-operated valve controls the flow, and many engines have a cooler to lower the gas temperature. The engine control unit checks the flow by watching the intake pressure, air-flow reading or a position sensor while it opens the valve. If the change is smaller than expected, it stores an insufficient-flow code.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Carbon in the EGR valve or passagesThe most common cause, especially on diesels and direct-injection petrol engines.
- EGR valve stuckThe valve does not open as far as commanded.
- Faulty EGR position or pressure sensorThe control unit cannot measure the flow correctly.
- Vacuum or wiring faultOn vacuum-operated valves, a leaking hose or a faulty solenoid.
How it is diagnosed
- Command the valve with a scan toolMany scan tools can open the EGR valve and show its position. Compare commanded and actual position.
- Inspect for carbonRemove the valve and look at it and the passages. Heavy carbon must be cleaned or the parts replaced.
- Check the sensorsCheck the readings of the EGR position sensor and the intake pressure sensor.
- Clear and test-driveAfter the repair, clear the code and drive at varying loads to let the monitor run.
Questions about P0400
Is P0400 the same as P0401?
P0400 is a general flow fault. P0401 specifically means too little flow and P0402 too much.
Can the EGR valve simply be blocked off?
Removing or blocking the EGR system breaks emission rules in most countries and can fail an inspection. It is not a legal repair.
Does driving style affect EGR clogging?
Yes. Many short trips and long idling increase carbon build-up. Regular longer drives at higher load help.