You wanna find out what it is?
Remove the downstream o2's and start the car. (not permanently, just a test) If the car runs the same, then the "clog" (if there is one) is before the downstreams, if it clears up, then it was after the downstreams, and the exhaust used the new place to leave.
You could have your emissions tested to definitively determine the cat's health.
Ray
