Anti pollution system fault

[Mxrph8]

Hi there, my ds3 recently came up with the anti pollution system fault, it is a 59reg 1.6 thp petrol engine, what is usually the cause of this?

[Chris_Blue]

not burning fuel effectively resulting in the coking up of inlet valves, maybe Cat converter, sensors dirty

How many miles it done

Needs a walnut blasting at a guess