When my car had that code failure, I did a lot of googling. And, a lot of mechanics said in most cases it is not the cat itself, but the sensors.
A review. There is no direct way to test the cat. So, there is a sensor at the input and one on the output. When the car is cold, the convertor cannot work. Once it is hot, the computer expects the input to have crud, and the output to be burned clean. Thus, there will be a difference between the two sensors.
If the cat is bad, there will be no difference. If the output sensor is bad, it may not show a difference. If the input sensor is bad, it may not show a difference.
It seems most mechanics throw the most expensive part at it. Then, when that doesn't fix it, sell you the sensors.
I bought a scanner which supplies sensor data. I got an EQUUS 3140, more than you need, because I live in Mexico where there are beau coup old cars, and the 3140 will read OBDI on some makes. If you hook up and note the normal readings when the car is working, you will have a better idea if this fails later.