Not sure I would be trusting your scan tool data. In order to get a p0420 your rear o2 would need to have fluctuating voltage which would indicate the cat has failed (rear o2 mirroring front AF sensor). Not to say it isn't possible but I have never seen a gasoline obd ii car have a p0420 and not have it be a bad cat. The manufacture is pretty tight on having all other monitors check and pass before throwing a p0420 (think about it, it's an extremely expensive part with quiet a long warranty so the manufacture does not want to be covering it unless they absolutely have to). Diesel is a different story, if the regen injector clogs and doesn't spray fuel into the cat to get it hot enough then they will throw a p0420 but gasoline doesn't have any systems like that to fail before the cat.
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