Get the codes read, it could be a number of things. How long have you had the car and what sort of driving do you do? The DPF could need regenerating, this can be done by starting it through one of the code readers available quite cheaply on the internet. Otherwise you need to drive the vehicle at a constant speed of 60 mph or above for about 30 mins to start it whilst driving. One of which is the C110 BMW scanner cost around £30 delivered on Ebay