A DPF cleans itself by burning off soot, and it needs a steady run with a hot exhaust to do it. A lot of Liverpool driving never gets there. Queens Drive, the dual carriageway ring that loops from Breeze Hill round to Aigburth Vale, is lights and junctions most of the way, and the school run or the trip to the shops is over in a few miles.
The motorways help less than you would think. The M62 ends at Queens Drive in Broadgreen and carries on as the A5080 into city traffic, so a commuter gets a few miles at speed, then crawls. Queues for the Kingsway and Queensway tunnels, and the lights on the East Lancashire Road out through Walton, add more idling. The soot builds faster than it burns, a regeneration starts and stops, and the warning light comes on.
A clean clears the filter, and the diagnostics show whether a failed pressure or temperature sensor is making it block faster.