Ok, I did the AFm mod on a soare AFM that I had and put it in, after a couple days I took it out because it was eating too much gas and i found out with my mods it probably wasn't doing anything that much anyway. Since then I have put the stock one in and reset the ECU. Now whenever I try to start the car when the engine is cold it sputters and sometimes backfires softly. It also hesitates for the first couple seconds until the engine warms up. Once the engine is warm it runs great. The exhaust smell is somewhat stronger also(I think, not quite sure because I didn't pay much attention to it until now). I'm thinking I screwed up the O2 sensor by running it too rich?(and maybe the cat. too?) Would a bad o2 sensor be causing these problems?