P0420 Engine Code 2016 Subaru Crosstrek

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About a year ago the check engine light came on on my wife's 2016 Crosstrek. At the same time the cruise control light came on in a blinking fashion. I took it to the dealership where we bought it and they said that it needed a new catalytic convertor for $2,700. I said I would think about it and took it home. I was a Chrysler auto mechanic 50 years ago and a gearhead. I replaced the convertor myself for about $625. After turning the engine light off, it stayed off for about 2 weeks and then came back on, then would go off for a few days and then come back on. My OBDII reader told me it was a P0420 code-catalyst system efficiency below thresh hold bank 1. The next thing I did was replace the downstream O2 sensor but it did the same thing after resetting the light. Next, I changed out the upstream O2 sensor and again the same thing after resetting the light. Had I let the dealership do the convertor, I believe that I would have the same problems with them. What am I missing? I don't know of any other sensors that I could change out in the exhaust system that would cause this code. Any ideas or help would be appreciated. Thanks.