03 GTP not making boost

Hey lads my buddy has an 03 GTP, he has only had it for a couple of weeks everything has been working great with the car, this past friday we went to Ottawa and notice he make at the most was 2 pounds of boost when passing cars, I dont no the proper name but could there be a vavle on the supercharger that is stuck open and all the boost is going out the by-pass vavle???? just a hunch yea the engine light is on but does not flash the prev- owner said he did have it tested and it was the fuel cap ???