I have a 2002 Vectra 2.2 SRI 150. The previous owner had replaced middle box of exauhst with a straight through pipe so I had to replace the cat after a month, because he stuck gunk on instead of replacing the gasket between the cat and manifold.

It's much better but power seems to come and go. The head was tested and gasket was fine, so I was told to clean or replace the air intake meter. I am also having a problem with the display, which now mostly says stopwatch 0min 0sec instead of useful information.