I have a 2007 cobalt with a cold air intake a stage 3 kit from cobalt addiction (2.7" pully, 60lbs injectors, 2 step colder spark plugs, a vibrant headers downpipe high performance cat combo and a GMPP cat back exhaust.

My issue started a few months ago when I would get a random cylinder misfire and a P0420 code, Catalyst System Efficiency Below Threshold (Bank 1). I just thought that the random misfire was caused by the P0420 code being thrown, but more recently I have been throwing a P0303 code which is a cylinder 3 misfire. I noticed this code when I went to get gas and after filling up, 93 octane always, my car started reved high and then went low and stalled.

I have checked all my spark plugs and they were all fine, but ended up checking them to be safe. Next I checked my cold air intake for any holes causing unwanted air. I still need to check my fuel injectors and if that doesn't work possibly change out the cat (thinking that may be bad)

Has anyone ever encountered such a problem like this before? I haven't been driving the car as fear that I am messing it up more. I don't want to take it for a compression test, just afraid that my 3rd cylinder won't have any compression and then i'll have the pleasure of buying a new engine.

Please let me know if anyone can offer any advise/assistance with this one.