Troule code P0420

Have someone check the 02 sensor wave patterns on a scope or for being out of spec. In particular the front one. Converter is probably fine, but further testing should be done. If testing was done, not just a conversation, then you'll need to have faith in one of the techs. That code points to bank1-02 sensor. The front sensor evalutes gases before entering the converter and the rear one evaluates it after the converter