One of our good mechanics stated on one of many discussions we have on cats, to find a mechanic with a graphing scanner who can look at the sensors and tell you with high certainty if the sensor is bad.
there are mechanics out there who will tell you that if the cat efficiency code is enabled, the cat is bad. Maybe; maybe not. (I say, probably not.) Get a mechanic who can tell you instead of throwing the most expensive part at it.
IF a mechanic tells you there is no way to know for sure, he may be lying; he may be dumber than a bucket of rocks. Either way, you need a different mechanic. But, he who has a graphing scanner can tell you for sure.
I had two bouts of bad cat efficiency on my US Sienna, which I drove to 220,000 miles. In both cases it was fixed with new sensors not new cat.