Fault codeP0401

MODERATE

Exhaust Gas Recirculation Flow Insufficient Detected

What this code means

The EGR valve is not recirculating enough exhaust gas into the intake, often because it is clogged. Nitrogen oxide emissions rise.

P0401 is a generic code: it means the same thing on every make. P0401 belongs to family P04xx — Emission controls. Its page explains how to read a code and groups all its codes by part.

Exhaust gas recirculation (EGR) valve

The EGR valve sends part of the exhaust gas back into the intake to lower combustion temperature and nitrogen oxide emissions. The control unit opens it according to load and checks its position with a sensor built into the valve.

Symptoms

Often no noticeable symptom beyond the light. Sometimes pinking under acceleration — the engine runs hotter without recirculation — and, on diesels, higher nitrogen oxide emissions that fail the emissions test. The control unit commanded the EGR valve open and did not see the expected effect.

Common causes

EGR valve caked in carbon, stuck closed — the dominant cause, especially on diesels
Blocked EGR passages or cooler
Faulty differential pressure sensor or valve position sensor
Leaking vacuum control solenoid, or a holed vacuum hose
Damaged valve wiring or connector

How to diagnose it

Remove the valve and look: on a diesel that has done a lot of town driving, it is often blocked by a hard black layer. Careful cleaning with a dedicated product sometimes restores it; if the seat or shaft is scored, replace it.

Clean valve but persistent code: check the vacuum circuit on pneumatic valves, or the electrical control on the others, then the differential pressure sensor. Then clear the code and drive: on many vehicles it only returns after several cycles.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Commanded EGR
Actual EGR position, valve 1
EGR error
Intake manifold pressure

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

Repair cost

Cleaning: €0 to €100 depending on whether you do it yourself. New EGR valve: €100 to €400 for the part, €200 to €600 fitted. A blocked EGR cooler costs more, often €300 to €800.

How to read this code

PPowertrain
0standardised code
4Emission controls
01P0401