The PCM judges catalyst efficiency by comparing upstream and downstream oxygen sensor readings. The U/S sensor is also used to adjust fuel trim, but the D/S sensor is just for monitoring the catalyst IIRC.
Replace your Bank 2 (front) downstream O2 sensor and see if the code returns. I had P0420 a couple of times last year, but it hasn't returned in over a year since changing the (back) downstream O2 sensor.
HTH!
