The Mercedes arrived with P0401: EGR flow rate too low. The EGR had already been replaced and the DPF cleaned, yet the fault returned. That history mattered: repeating the same repair would only repeat the uncertainty.
What the code could not tell us
A fault code describes what a control unit has detected. It does not prove which component caused it. Testing showed the replacement EGR initially responded, then failed repeated actuation. The exhaust-side actuator operated, while pink coolant residue near the EGR cooler added another useful clue.
The evidence-led recommendation
The recommendation was to fit a genuine Mercedes-Benz EGR, then clear adaptations, test, road-test and rescan. The cooler was not automatically condemned. That distinction matters: inspect the wider system, but replace only what the evidence supports.
The lesson
If a Mercedes P0401 returns after previous EGR or DPF work, do not assume the filter needs cleaning again. The operating history, actuator response and related system evidence need to agree.