Fault code info

Hello ppl. Got a management light on today driving to work. P0420 catalyser efficiency fault apparently. Just wanted to know if it is most likely the cat or just a sensor fault. Cheers

So a quick update. Used my torque program to graph the O2 sensors. Sensor 1 should be a nice waveform from around 0.1 to 0.8 millivolts. This was fine. Sensor 2 however, the one after the cat should be around 0.5 millivolts and steady. Oh dear at 2000 rpm it just mimics sensor 1. Duff cat it is then. I'll leave it for now as I don't have time to fit a new one just yet. Sensor 2 doesn't effect the running anyway.