There are better experts on emissions than me but here is my opinion:

The O2 sensor and the cat do 2 different things, so to say you need both is a little fishy IMHO if you just have the one problem. As was just said, the O2 sensor might well solve your problem. It is cheap and you can DIY. I had the same problem with my Toyota, high CO, really high like yours. I changed the cat, NO IMPROVEMENT. I change the O2 sensor, CO went to 0. I would put in the O2 sensor and get it tested again.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra