engine diagnotic warning light?

I have had a peugeot 307 sw se 2003, for the last 2 years after the 1st year I had to have a new cat. In the last few months it has developed a judder when I start the car and sometimes stops. Yesterday the engine diagnostics warning light came on and has stayed on. Reading through the q and a it seems as though it could be the FAP?

Has anyone got the answer before I take it into a garage and get mugged

Thanks