The sensors in the cat are not used for anything except cat efficiency testing (comparing the delta in O2 readings against the upstream). An efficiency code is expected in this case with test pipes (UpRev tune can delete these codes). If it is a different code you should let us know, I got a code for the upstream sensor (the real O2 sensor) when I blew out my exhaust flange gaskets where the header meets the test pipe. The real O2 sensor is installed in the bottom of the factory header.
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