So recently my 07 baseline sky has been acting crappy. First off it will long crank and sometimes not even start at all until 2 or 3 tries. Then when it finally does start up the RPMs will run at like 200-500 and die if i dont rev it up. The codes thrown are P0496 which what i can find seems to be the pressure valve not opening or closing when it should or the sensor messing up. How can I best diagnos which part exactly is bad? Also I just replaced my O2 sensor on Bank 1 sensor 2 and am now having code P0420 for catalyst bank 1 sensor 1 as well.