Thanks for comments.

What I was really wondering was whether anyone has had to take an almost new car back to the dealer within the first 12/18 months to have the DPF "cleaned" by them. In my mind it is difficult to assess whether manufacturers are making progress in improving the technology because the fault is unlikely to show up in the first couple of years or so of the Car's life (I think, but would be in terested to hear otherwise).

I have seen factory bulletins about DPF which state that the car must be travelling at above a given speed (for example, 38mph in the Vauxhall bulletin I saw) to start a 'normal' regeneration.

I believe this is to help minimise heat build-up in the engine bay when the DPF canister is being initially heated to the start temp for the regen.

However, once the regen has started it will usually continue, irrespective of speed, until the sensors on the DPF determine that the soot loading has dropped sufficiently. The ECU can keep the DPF matrix temp high enough by throttling the intake and adding extra post-injection fuel.

On my Mondeo and Volvo, the engine cooling fan is always on when a regen is happening, again I presume this is to shift underbonnet heat.

Obviously if a dealer forces a regen in their workshop, this overrides the ECU and presumably the main engine fan runs during the process.

EDITED to answer the OP's extra question -- I think dealer-forced regens are getting rarer. There was a spate of these with VAG cars but it seemed from various VW / Audi / Skoda / Seat forums that they also had a spate of duff DPF pressure sensors, which were telling the ECU that the DPF was full of soot.

Having had a Mondeo IV for over 3 years, there were very, very few DPF problems reported on the owner's forums, and no dealer-forced regens that I can recall.

Edited by craig-pd130 on 02/03/2013 at 19:58