P0401
ModerateP0401 — Insufficient EGR Flow (AGR/EGR)
Description
What the Code Means
P0401 sets when flow through the exhaust gas recirculation valve (AGR, Abgasrückführung) is too low. The AGR system routes part of the exhaust gas back into the intake to lower combustion temperature; when the control unit sees actual flow below the expected value, it stores this code.
The code shows up on Mercedes, BMW, Ford, and PSA vehicles and is often tied to carbon buildup (Verkokung) on the EGR valve and the recirculation passage. The source lists P0401 together with P0402, the code for excessive flow, which points to the EGR valve and its passages as the shared problem area.
Symptoms
Symptoms
- Increased carbon buildup (Verkokung) on the AGR valve and the exhaust gas recirculation passage — per the source, this symptom is linked to P0401/P0402.
No other symptoms are given in the source.
Causes and fixes
Diagnosis and Repair
- Connect a diagnostic system — the source lists XENTRY, ODIS, and ISTA.
- Run an actuator test (Stellgliedtest) to check AGR valve operation.
- Perform an air mass measurement (Luftmassenmessung) — this separates the causes: a faulty valve itself or a problem in the air path.
- If carbon buildup is confirmed, do a proper cleaning (fachgerechte Reinigung) of the AGR valve.
- After cleaning, run an adaptation (Adaption) of the system.
Common diagnostic mistakes, specific flow values, and torque specs are not given in the source.