By the way just as a thought, I thought that maybe rather than making a lot of noise around the house…. You could use a compression tester with a 18mm adapter on it. IF the upstream sensor locations have one that is higher than the other bank. I would bet that the high one is the offensive cat. If bank one and two are high, and the down stream is high, I would bet on the third cat. IF bank one and two are high and the down stream is low, I would bet that the bank 1 and 2 cats are plugged.
But, a two or three hundred PSI gauge probably wont be sensitive enough to tell the difference.
Sounds to me like I just though of a new E-bay product line-up!
A plugged up cat does not make the O2 sensor go bad. It just will make the down stream sensors switch faster than the upstream O2 60% Tolerance. The down stream 02 sensors are only used to test the efficiency of the cats and are not used for fuel trim. Knowing the ways that MIL eliminators work, and back pressure works, I GUESS that it could make the car run richer not leaner, as well as hotter. But there is a bunch of things that would change in that whole setup so I really prefer not to guess. I am one of those guys that won’t carve anything in stone unless I see and test the results first. Hands down the best way to check the 02 Sensors is with a graphing scanner first. Then check the wires to see why the sensor heater is open or the thing is stuck high or low.