2000 mazda protege

Well, i seem to be in the right place. About two weeks after i bought my car, my CE light came on. Did the free Autozone check and it's a P0420 code. "low catalyst threshhold, bank one" Now i've searched some forums, but i haven't seen an actual answer to the problem yet. The responses i've seen are either the 02 sensor or the cat conv. But no one has seemed to fix this problem, or at least post the repair. My hope is it's the 02 sensor because it's a heck of a lot cheaper than a cat. Anyway, any help here would be appreciated. I really do enjoy this car and would like to clear up what seems to be it's only problem. Thanks
Josh
2000 Protege, 1.6L, 100K miles. See if you can get a freeze frame. if you see the light came on when you was doing more than 20 mph you most probely have a deteriorated cat or a malfunction exhaust system.