P0103 Fault on 2003 Volkswagen Golf 1.9 TDI: Diagnosis and Repair Guide

My 2003 Volkswagen Golf 1.9 TDI started with rough running, a noticeable rattle, and eventually died. After restarting, it produced a large cloud of smoke. I managed to drive four miles to refuel before the issue returned. On subsequent restarts, there was hesitation and the P0103 fault code appeared almost immediately. The vehicle experiences minor intermittent power loss, which resolves after clearing the code. I'm now wondering whether a fuel filter clean or replacement is needed — or if this points to a more serious issue like a faulty mass airflow sensor or intake system leak.