i replaced the front o2 sensor because i got an ses light that said something about low efficiency in bank one. i have no clue what a bad o2 sensor looks like, but in my opinion it looked bad! i was just wondering before i go spend the money on a new rear o2 sensor if it should be replaced also. the ses light will come on and go off, hopefully that will fix the light?
I would check out the catalytic converter. We were getting some SES lights also for the same thing so we took ours to a dealer and paid them to diagnose the problem so I didn't spend money on stuff I didn't need. The cat was partially blocked. We snagged a new Magnaflow one off of Ebay for $50 shipped