I had the same P0420 code come on at 100,000 or so on my 03 GT. A local shop put in a new cat and an 02 sensor. The code away for about 1 yr and came back. Shop put in another o2 sensor for free, but the light came on a week later. Shop put in another cat for free, and the light came back a few months later. Problem was the aftermarket cat was not doing a good enough job.

Permanent fix was a piece of black tape over the light.