[mrneuro]

bank 1 - the bank of cylinders that has cylinder #1
sensor 2 - the aft/rear sensor (though it could be the other one on that bank that is bad and the computer getting mixed up)
http://www.allpar.com/fix/codes/new-codes/oxygen-sensors.html

 

[ImperialCrown]

If you are replacing the 1/2 O2 sensor for fault code P0139 (O2 slow response), it may not be from the O2 sensor being bad. There is a TSB # 18-031-04A for a PCM flash that corrects this false code setting. This TSB also installs other PCM software enhancements. I highly recommend it as the next step if it hasn't been done yet.
You can read about it here:
http://www.lxforums.com/board/f62/tsb-18-031-04a-14343/

 

[Markoglide]

Thanks for the info. I'm getting a code for high voltage not slow response. Looks like ill try the left downstream sensor

 

[ImperialCrown]

Yes. Driver's side, after the cat.