this relates to your particulate filter which gets blocked up with soot particles,it may be worse if you do a lot of town driving,you could try and give the motor a long run preferably on a motorway with high revs to get the exhaust really hot to burn off excess carbon deposits in the filter,if that does no good then it may need to go onto a relevant code reader to force a regeneration of the filter
cdc, November 2012
