This vehicle eventually came back with the same fault. Obtained a replacement cat from Unipart and noticed that the new one was far better quality than the previous one they supplied ie ceramic core as opposed to metal. That was a couple of months ago so it seems all is well now. Have read all replies and I understand how the rear sensor monitors the efficiency of the cat but as Pete Mutlow says I still don't under stand why a cat which is producing a perfectly acceptable set of exhaust gas figures should throw up the P0420 code. Cheers Dave.