problem with depollution system on 407?

Is the car petrol or diesel?
Have you been using cheap supermarket fuel?
Google depollution system peugeot, you'll get a lot of hits.
There are lots of reasons/solutions for this, eg

http://www.peugeotforums.com/forums/showthread.php?t=347

http://www.peugeotcentral.co.uk/ftopic-2035.html

Lots of info on the various peugeot and 407 forums.

warthog
August 2009

http://www.theaa.com/motoring_advice/fuels-and-environment/diesel-particulate-filters.html

Peccavi
August 2009