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I am not at work right now so I do not have access to all of my manuals and stuff but typically if you are posting a rear O2 sensor code it is indicative of a bad cat. The rear sensor is there for one reason only and that is as a reference sensor for the front one. When the rear sensor starts to read like the front one it interprets that as the catalytic converter is not doing it's job. The sensor signal varies with the amount of oxygen in the exhaust gas content and the cat uses that O2 to catalyze the other gases. So the O2 levels going in tend to peak and valley and when they come out they are much more of an even level so the front sensor typically bounces up and down and the rear is a flat line, when the rear starts to bounce up and down like the front then it determines that the cat efficiency is falling off. Fenix may post other codes for the rear O2 (why I don't know) but the only LH and Motronic codes I have seen are for the rear O2 are for catalyst efficiency. Hope this helps a little.

Mark