A non switching downstream 02 could either be a bad O2 or a defective converter,although i'd expect a code.Sounds like you have a decent scanner at your disposal,With the OTC in datastream check it's going into "closed loop".
I had a customer a few months back that had an issue where it wasn't going into closed loop and i ended up having to replace the ecu, his initial complaint was poor idle and bad fuel consumption,although this is rare as it never once threw a code whereas it should of,but that i'm putting down to a damaged computer. He came back a few days later to say he'd found out his son had tried jumping his friends car and had crossed the cables more than once in attempting to do so.
Good Luck
