CAT O2 Sensor Question

I was helping a friend with a problem on his XJ today. His 97 is idling erratically. He has replaced the MAP and O2 sensors, plugs, cap, rotor, and ran a scan. Nothing is showing on the scan and the new parts did nothing to cure the problem. In the process of working through everything he stated that he thought the previous owner may have modified the CAT. If this were the case could the O2 sensor at the back of the CAT be transmitting a bad signal creating the problem.

not likely. it's only function is to monitor that the cat is doing something. the pcm compares to the rear o2 readings to the front, and decides if there's enough difference to not scream "P0420, P0420!"
i actually unplugged mine on the '96(also obII) when it went bad, just to see what would happen. made absolutely no difference in running, driving, or gas mileage.
more details as to how exactly it's running and idling?

IAC dirty, TPS going out?

I haven't heard it or looked it yet so I really have nothing to add at this point. He just called me for advice and I figured I would toss it out there and see if anyone had a suggestion. My take was much the same as yours even if the cat was altered it probably would not affect much if anything at all. I have seen this with a bad ground on a Ford FI....might be the problem?

What does the exhaust smell like? If you put a voltmeter on the O2 sensor, what does it read?

Could be a bad ground or just about anything else.