During the past year or 25k miles I've had problems with an engine light with code p0420 which is cat efficiency low. I was told to replace the secondary o2 sensor and clean the mass airflow sensor. I replaced them both while I was doing it. That seemed to take care of the problem until about a week ago when the same code fired. I always check with the local dealer to see if there are others with similar issues, but they're never much help. I am not against replacing the dual cat system, but want some advise before I drop any more money at this. Any help would be appreciated
Grif