I don't recall exactly how I did it, but I did it without taking any pipes loose! I think I used a combination of the 02 socket, and an open end wrench. You can only turn it very small amounts at a time, but each bit is progress. The one " trick" I did employ..... I used a battery hold down bolt ( a long rod with threads on on end, and a bend on the other) to plug the sensor back in. Unplugging isn't a big deal, but plugging it back in, I took said rod, and hooked it over the top of the upper part of the plug and pulled slightly as I pushed the bottom portion of the plug up into it. You have to be careful not to tear up the insulation on the wires, but it worked like a charm. Just take your time and think it thru. It does take time, and your forearm will cramp a bit from the odd angle your arm will be in for the time it takes.
I'm assuming you had the cats checked to see if they were plugged up?