After re thinking my input, and with the above input, post cat sensor shouldn't be able to have any affect on fuel trims, it would be throwing an 02 fault . It's only really there to check your cat is working. I suppose it's a possibility that it could be the previous cat lambda. But really you need to so some logs, and figure out if it's rich because the ecu is correcting to add more fuel for some reason . Or if it's putting in the fuel requested with no corrections going on, then it means you're missing the air aka boost leak