exhaust problem

can anyone help me with this car, ford galaxy 2.3l 51 reg, the problem is that this code keeps coming up and making the engine light come on, the car has got a new caylist fitted but not a ford spec. the code is p0420 which is a caylist related. i cant work it out? the engine is running fine and no other codes come up. if anyone had this code before please tell me what the fault is? thanks
marko

It's the downstream heated oxygen sensor.