So i Have an 06 2.2l 2dr automatic trans cobalt. All stock except k&n filter. Car has 193000 on it. Great car! I am a some what competent diy er with all hand tools but thats about it. Currently having issues with #1 cylinder miss fire, and catalyst system below threshold. started with plugs cause it needed it no help read up on here other stuff to try-cleaned throttle body(was very dirty but cleaned up easy),checked compression 165,170,170,165, no vacuum leaks or intake leaks, found cracked stock exhaust manifold. Also o2 sensor #1 is an odd looking shape like it has gotten way too hot! So my questions are #1 does anyone have a stock 2.2l exhaust manifold w/o a crack in it they want to get rid of? #2 is this whats causing #1 to miss fire? #3 did crack in manifold melt front o2 sensor or do i have other issues? Car is running good slight decrease in mpg no overheating or any other problems that i can tell. i have a scan tool and can watch my o2 sensors live data. Both front and back sensors are jumpy (indicative of a bad cat) but am wondering if all is from the cracked exhaust manifold and bad o2 sensor(the one that looks kinda melted) throwing everything off? Any help is greatly appreciated. Hope its not a bad cat too im in Ca. and dont even want to know how much new cat is.