The warning light activated on my S204, prompting me to use my Icarsoft Pro to diagnose the issue, which indicated a DPF reading of 400%. All other parameters were OK. This is peculiar, as my Mercedes is mostly utilised for long-distance travel. A DPF pressure difference sensor is once again required, I believe. Indeed, Mercedes has concealed it in a challenging location, since it has been replaced before. I changed it and then illuminated the ends of the previous sensor plastic hose to see that they were obstructed by a white sludge, which I presume is a result of condensation, given the car is used seldom for lengthy excursions. I cleared the code and had a drive, during which a regeneration started immediately. After around 30 miles, the engine light illuminated, prompting me to stop and examine the DPF soot level, which remained at 400%. I recorded that I changed the DPF using my Icarsoft Pro, and all readings are now OK. It seems that as the weather becomes colder and wetter, condensation has reached the sensor, resulting in a misleading measurement of 400%. I could likely have managed by only cleaning the sensor tubes; but, replacing them is somewhat cumbersome, so I opted for a new one instead. It is advisable to examine this sensor replacement, since the DPF was not at full capacity. I hope this information becomes beneficial to someone before to incurring substantial expenses at a garage.