P0420 is a Catalyst System Low Efficiency code. Gm's service procedure says to check ur exhaust for leaks and make sure ur rear o2 sensor is not damaged and working properly. if this checks out ok then to replace the cat. converter. If ur just trying to pass emissions u could remove ur rear o2 sensor, tie it up underthe car and plug the hole, then disconnect ur battery to reset the light this should make it think that the cat is working properly. Good luck