I had the same P0420 code a few months ago. I changed the O2 sensors and still had the same code which lead to a bad cat. Apparently, the only cat that will work on the GT is the OEM stock item...$700.00 and a week later, I finally get my car back and haven't had a code since. Wish I had found this ...

I have 122K on my '03 GT and just starting to have a hint of slippage when downshifting to accelerate hard. I bought mine while stationed in Germany and by no means babied it. It visited the speed limiter at 138 mph almost daily and my wife isn't the easiest on the clutch. I've experienced the "...