I have a 99 CRV with 117,000 miles. About 4,000 miles ago the check engine light came on,so I took it to the shop and they said it was the cat converter...so we put one in. About 2 days later the check engine light came on again! We go to the shop again and they say "cat converter is bad!" So we go back to Monroe muffler and they put another cat in....Well you guessed it ! 2 days later the check engine light comes on again. I call Monroe and they say to change the rear 02 sensor. I go out and get the sensor put in.A week later Guess what??? check engine light on again!! So I go out and buy a code reader and it gives me the code number 0420 which translates to "cat efficiency below threshold Bank 1"What does this mean?? So for the hell of it I put in a front 02 sensor and 100 miles later the check engine light comes on again! I really have no clue whats left....2 new 02 sensors and a new cat twice....Any help would be greatly appriciated!

Thank you very much
Edward