Here's my guess.
The two O^2 sensors [before & after cat] are either confused, or one isn't working properly. Though it seems too soon for an oxygen sensor to wear out.. :dunno:

:thinking: Has your MPG plummeted?
Is it possible for the two set of wires to be plugged in wrong-way round (the after cat sensor accidentally plugged into the the pre-cat sensor's harness or vice versa)?

The code number will help for sure.

Dave