DRTYTC

hi my name is Jon I'm a newbie. I grew up with hemi v8's but I loved the way the scion tc looked and drove. I noticed a change in sound when i started up but I am not 100% if it's the CAT.I've been told to try to change out the rear o2 sensor.
any answers or theories on what could be the problem??
Much obliged

DoinItWright

P0420 is " catalyst inefficiency ". It means that the computer does not detect enough of an emissions difference between the first o2 sensor ( the one in the manifold ) and the second one. Means the first cat is shot.