Simota, I think the scan gauge is just what you need to do your own basic diagnostics. With it you can check and re-set fault codes, check the air fuel ratio (as I mentioned in my PM, make sure to set it to show the right A/F reading, see:
Air fuel code for scangauge) and you can check the air intake temperature. Good luck getting it all solved. And another thing, I'd tell the dealer that joy rides in your car cost $120 each, so you don't pay for the $120 diagnostics they did for you...