Remove the post cat sensor and tie it up under car where heat cant damage anything. Plug the hole in the exhaust with an old sensor or something. reset codes and see what the post cat sensor is reading or if fault reappears should be reading fresh air therefore will lead the ecu to think that the cat is working efficiently. This will rule out the sensor. If the front O2 sensor is reading correctly and there are no other overfueling issues carry out emmissions test to see what the cat is doing if readings are high the cat isnt too hot. Find that the cheaper cats take a long time to warm up and arent as efficient causing the post cat sensor to pick up a bad cat reading on warm up.