As said......can be a multitude of things, so is near on impossible to give you an easy fix, I've been there, and with everything else, it will be a process of elimination. I went through a process of checking and cleaning every connection and fuse, new battery, new earth cable and still an issue. Garage checked and fault code saying it was the ecu. Almost went for a brand new ecu which the garage recommended but just before, based on lots of research on here, replaced the mass air sensor and hey presto, it never threw up a warning again. Now, that's just my experience and just goes to show how much of a common and minefield of an issue it can be, because for someone else it could have been fixed by any of the initial options or indeed it could have needed a new ecu......good luck!