P0420 Catalyst System Low Efficiency= ECM, PCM.

Yup you need a new Cat. Bill will probably chime in. Gm warranty their original cats for 10 years or 80k so your cat would be out of warranty. Easy and cheapest thing to do is either replace it w/ a universal cat such as magnaflow and have a exhaust shop weld it on or if your state doesn't have emissions you could just put a pipe in it's place.