P0401: Exhaust gas recirculation (EGR) flow insufficient
The exhaust gas recirculation (EGR) system is flowing less exhaust than expected.
Updated
What P0401 means
The EGR valve leads a little exhaust gas back into the intake. That lowers the combustion temperature, and with it the nitrogen oxide (NOx) emissions.
The engine computer opens the valve and checks the effect, for example on the intake pressure or the air flow. If it sees less than it expects, it stores P0401.
Can I keep driving?
Check soon
Probably OK to drive short-term, but have it checked soon. Fuel use or emissions may be worse.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
The car usually drives normally, sometimes with a little less power. Emissions are higher, so have it fixed before the next emissions inspection.
Symptoms
- Often just the check engine light.
- On some cars, less power, a rough idle or hesitation.
Common causes
- Carbon-clogged EGR valve or passages. Soot from the exhaust builds up, especially on diesels and cars used mostly for short trips.
- Sticking EGR valve. The valve can’t open fully, or doesn’t move when told to.
- Faulty EGR position or pressure sensor. The valve works, but the computer can’t see it.
Typical fixes
- Have the EGR valve and its passages inspected and cleaned of carbon.
- Replace the valve if it sticks or doesn’t respond.
- Have the position or pressure sensor and its wiring checked if the valve is clean and moves freely.
- Clear the code after the repair. Under emissions readiness, the EGR self-test shows when the car has re-tested the system.
Questions
Can I drive with P0401?
Probably for a short time, but have it checked soon: the app’s advice is that fuel use or emissions may be worse.
Does P0401 mean the EGR valve must be replaced?
Not necessarily. Carbon build-up is a common cause, and cleaning can be enough. A valve that sticks or doesn’t respond, or a faulty sensor, needs a repair or a replacement part.