[onewhippedpuppy]

Usually it's easy to check pressure at the rail, maybe give that a shot to eliminate fuel supply as a question. Ever hear a rattling sound in the cat area? Often times the core stuff (tech jargon) starts to come apart when they're shot, and it sounds like marbles rolling around in the cat. Try banging on it with your hand. Not the only way they wear out, just one of them. I'd be suspicious because of the O2 sensor code, very well could be something else causing it to throw the code. Maybe try pulling the pre-cat sensor and see if it makes a difference with the reduced backpressure?