i can only tell you that it started with P, and all he said was that a pipe could need changing as it was squashed slightly.. it was only an air pipe somewhere at the rear/ bottom of engine bay. my partner (whose account im using) changed this pipe, but i dnt feel the problem has gone altogether as i sometimes feel it losing compression and the engine management light has flickered a couple of times.. Garage did say could be trial and error. also he said that wouldn't be a sensor or it would have pin pointed the problem on the comp.