What is the code that you are reading?
I'd do the spark plug non fouler trick. You'll need two, drill one out so the O2 sensor threads into it, then thread that into the other(undrilled) non fouler. Then thread the entire thing into the bung in the exhaust pipe.
The good news is the rear O2 sensor doesn't effect anything other than emissions control. So it won't effect performance.
Breathing in and out is good enough for me.