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