I've had an SES for the banks 1-4 pre-cat O2 sensor and a cat. converter on that side as well, so I switched out the O2 sensor a month ago and as of late, I've noticed that the car has some SEVERE cold stumbling that goes away very quickly.. even before I drive 1/2 a mile. It's almost as if someone hits a button and the hesitation just disappears. After that, it's fine, and it only occurs in the morning on a true cold start. I've read that people have attributed it to the MAFs, an O2 sensor, and have even called it normal, but could this be something due to a faulty cat. converter? I just wanted to get some opinions before I get it taken in for a check. I've also read that a Powerchip upgrade will do the trick, and that's a nice excuse to do it.