im cam a licenced diagnostic tech.
the p0420 is a catylist efficiency code.
I have seen this code many times on these engines and it has usually been the cat conveter.
a cracked exhaust manifold will also cause this code.
to check the cat...
montor the front and rear o2 sensors. if the rear o2 sensor is matching the front o2 sensor than the cat will need to be replaced
the front o2 should be toggling between .3 volts to .75 volts fairly quickly, while the rear o2 should be a steady slow wave volt range.
if the rear is toggling fast, that it finished.
this can be seen properly with a graphing scale on a diagnostic scanner.
you can use a multi meter volt scale but it will be a little slower to monitor as multimeters do not have a fast enough processor to catch every crosscount.
for me. it has always been a defective cat.
thanks and i appreciate positive ratings