There are many reasons for a P0420 (read the TSB article posted). Most times, it could be misleading as we usually jump to a bad Bank 1 catalytic converter. Do change the Bank 1 Upper sensor. If you cannot get a correct torque, just tighten as best as you can. Actually, I did not torque my O2 sensors to specs when I did my mileliminators for the lowers.
Here is the Bank 1 Upper sensor how-to from the Duratec Maintenance FAQ.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=v6maint&Number=1023736&Forum=v6maint&Words=replace%20o2&Match=And&Searchpage=0&Limit=2500&Old=allposts&Main=1023736&Search=true#Post1023736
