Frequency wise, it varies. It's based on the build up of soot in the system so there's a pressure sensor that tells the ecu when to do it. Depending on the type of driving you do, you might not ever notice it as no light comes on but on the odd occasion, if you come to a stop and the idle speed is high (about 1k rpm whereas its normally about 800rpm) then its doing it. Looooads of heat will also be coming out from under the car and thy exhaust tail pipe. At this point if your journey is over you can either leave it to idle for a while and wait for the revs to come back down (can take ages) or take it round the block. Personally I wouldn't drive it hard if you do as nailing a diesel causes soot, and its that its trying to burn off. Instead hang on to your gears a bit so perhaps when you would normally be in 4th at 30mph for example, sit in 3rd instead to keep the revs slightly up. I'm lucky in that I have an open 50 limit road near me so I go down there at 50 in 4th to keep the revs above 2k to burn it off quick then return home.

If the light ever comes on, it means the car can't regen on its own. Usually means you've either stopped the car too many times when its been trying and this means it needs assistance which is to take it for the drive I've described above. If the light does not go out then its dealer time, but that's a last resort.

This is taken from the leaflet the dealers have on their desks about the DPF, and I asked a few questions too.

For us Petrol wins we do around 8k a year and the 5 series diseasal we had was crap on long runs it was good averaging 47mpg not bad for a 2.5 weighing a few pounds incl the owner had it remapped and thats were the problems started although the vrs is seeming just as troublesome :wonder: but i like it its really growing on me and hopefully after a few things are sorted and it behaves for about 6 months i may remap it and get a miltek AmD Essex are offering the package for £667