P0420 code keeps coming back on my Mustang: What does it mean?

[v8ismyheart]

So, my Mustang keeps throwing the P0420 code, and it’s driving me nuts. I clear it, and boom, a week later, it’s back. I’ve heard this could be the cat converter, but before I drop serious cash, could it be something simpler? Maybe an O2 sensor? I’m hoping someone here has dealt with this before and can point me in the right direction. Thanks!