Steven Ward responds:

With regards to this issue, the point the Denso rep was making to me was this: if you’re replacing a cat or you have issues, then you need to consider replacing the whole set-up, especially if the vehicle is beyond 60,000 miles. To replace a cat without the sensors or to replace the one sensor without the replacing the corresponding one at the other end of the loop is a false economy and will raise more issues than it resolves.