I have a Citroen C4 1.6 petrol (2006) and the other day while I was driving the engine light came on with a message "Depollution system faulty". I connected the OBD-II to my computer to get the error code and it was P0350.
From that error I think my option would be that a wire is loose or something, because I didn't feel anything weird while driving. The problem is that I don't know where to look in the engine to check the cables are properly attached.