P0401 – EGR Flow Insufficient (CDI)
On Mercedes CDI, P0401 can drive secondary DPF efficiency codes and repeat soot loading. Fixing EGR flow often stabilises the whole system.
How it usually shows up
- Hesitation, smoke under acceleration
- DPF faults appearing alongside EGR codes
- Poor fuel economy
Most common causes (Mercedes)
- EGR valve sticking or restricted passages
- EGR cooler restriction
- Airflow sensor plausibility (MAF/MAP) affecting EGR calculations
- Vacuum/actuator control issues (older systems)
Practical test plan
- Check for other airflow/boost codes; fix plausibility issues first.
- Inspect EGR valve and passages for restriction; confirm commanded vs actual response if live data supports it.
- Verify vacuum/actuator control (where applicable).
- After repair, clear codes and allow adaptations to settle; then re-check DPF data.
Related reading
CDI DPF & EGR Behaviour
How EGR faults create secondary DPF issues.
P2002
DPF efficiency – common recurrence traps.
Tip: If you have multiple codes, start with the one that appeared first, then confirm with live data before replacing parts.