The recirculation system (EGR) returns part of the exhaust gases to the intake to reduce the combustion temperature and nitrogen oxides. The computer has opened the valve but is not seeing the expected change.
Low to medium. The car drives, but with diesels, emergency mode often follows, more soot and faster clogging of the DPF.
Mostly diesels, but also gasoline engines with EGR. Valve, cooler, ducts, vacuum control.
What does P0401 mean
The computer checks the flow through the EGR indirectly - by the change in pressure in the manifold, by the flow meter or by the temperature sensor. When the valve is commanded to open, and the effect is absent, it records P0401. Almost always it is soot, not electronics.
Symptoms
- Lamp "Check Engine"
- Less power, sometimes emergency mode
- Knocking (detonations) under load in gasoline engines
- More frequent regeneration or clogging of DPF (P2002)
| Can it be driven?
Yes. The code isn't urgent, but on diesels, don't leave it for long — more soot means more expensive particulate filter repairs.
Common reasons
- Scale in the EGR valve — the valve does not open enough. The most common reason for diesels.
- Clogged EGR cooler or ducts in the intake manifold.
- Vacuum control — burst hose or faulty solenoid (on older systems).
- Defective sensor for valve position or pressure.
- Electrical problem — coupling, wire (see P0403).
How we diagnose it
- We command the valve with diagnostics and see if the values change.
- We check the vacuum and the solenoid if the valve is vacuum.
- We disassemble and inspect the valve and channels for soot.
- We clean or replace — and erase the code with a test drive.