Possibly your diesel vehicle has an "Addblue" system fitted for the Diesel Particulate Filter (DPF). Or, if not the vehicle may need to be regenerated by running to operating temperature and then drive for 20/30 min with the engine revs at around 2500/3000 RPM. This should put the ecu into regen mode. If this fails you may need to see your local garage and get a full diagnostic check to determine whether or not the DPF needs to be cleaned or replaced or even check the sensors.