I have a 2010 Volvo XC60 T6 Showing P0420 with just over 200K miles. The sensors and Cat were later replaced. The code went away for a short (very short) while but keeps coming back. I drove over 200 miles to take the car to the nearest Volvo dealer who said there were other codes (P0172 & P0101) and they replaced oil trap and said the air intake had cracks which was also replaced. While driving home approx. 150 +/- miles the Check engine light came back on and the P0420 code came back. After that the Cat and 2 sensors were replaces. Almost 3 years later the code is still active but nothing appears to be wrong except the code will not stay off. I am getting 20 - 24 MPG. What should I look for next (I am thinking a different car but NOT a Volvo.)
Have you checked into the possible causes of P0420 other than the converter and O2 sensors?
https://www.walkerexhaust.com/support/tech-tips/diagnosing-catalytic-converter-p0420-code.html
https://www.way.com/blog/p0420-code-what-it-is-and-how-to-fix-it/
https://www.obd-codes.com/p0420
That's a common code, if you search the board and check the FAQ you'll find a lot of information.
Also, it would be best to stay away from dealers and find a good independent mechanic.
I have checked the links you sent me and All has been done. Cat, 2 sensors and 6 spark plugs and still the P0420 code comes back. At this point maybe I should go to Nevada to the despensory and buy my car some 420 and send the smoke into the system, I don;t know of any other way. From what I read the major issue would be not passing a smog test. Fortunately where I live does not require smog. My Volvo days are numbered. As previously stated this is the first and last Volvo. in 2010 the company was owned by Ford and now China so I am looking for a replacement auto.