Sporadic CEL P0420

I recently (about 2 months ago) purchased a '04 Prius with 122k miles on it and it now had 124k. The only problem that it seems to have it a sporadic P0420 cel that come on and off every once in a while (even while on the same tank of gas); I always make sure the gas cap is on tight. On my last fill-up I got about 50mpg, so nothing seems out of the ordinary.

I'm hoping to pick up a scangauge soon to read the o2 sensors, but was just wondering if anybody had any experience with this problem and what their outcome was.

Thanks,
Anthony

**Also, if anybody have a suggestion on how to fix the rattling center dash speaker that would be great.