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The only reason you get a code if you remove the cats is because all the rear o2's do is detect flow. To much flow = Cat problem. To little flow = Cat problem.
Since removing them will increase the flow, the "non foulers" pull the sensor out of the exhaust stream a bit, thus cutting the flow it senses.
I received the risers from Brenspeed today. But go figure! The "Check Engine Light" has not come back since Brenspeed walked me through the procedure of "reading and clearing DTC's" from my XCal2. I am going for inspection tomorrow for emissions only. Let's see what happens. And yes, they were only $19.99 for the pair.