We hav a Fiat Multipla JTD 52 plate. The cylinder head gasket was changed in May , since then the engine warning lamp has been coming on with code P0236 over boost valve pressure. There are no intake or exhaust gas leaks. MAF sensor has been changed, and the overboost valve sensor has been changed, however the fault still keeps reoccuring. Now seriously fed up as car is running in limp home mode. Can anyone help us? The garage has no idea what else could be causing the problem.