First thing to try is disconnecting and cleaning the male and female connectors of both O2 sensors. Clear the codes and see if that eliminates the P0420. If not, many DIY guys try hanging on one, and then the other new O2 sensors. If still no joy, it's almost definitely the cat gone bad, and there's no wiggle room on that. By any chance is your Aveo inside of 8 years from (new vehicle) purchase date, and under 80K miles?