Well, the p0420 is an 02 sensor for sure, but thats an after cat sensor DTC. Its sensing from the before 02 through the cat-converter to the rear 02. The rear 02 is either the issue or your cat is toast. The u0121 is a manufacturer issue and not a powertrain (p=powertrain, u=unibody).
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille