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/