Odds are what?

SES light on again, this time it set code P0420 Catalyst System Efficiency Below Threshold (Bank 1) Reset it and it came on again 3 days later, same code. OK Folks which would it be, O2 Sensor or Cat Converter?
Thanks.

mine is does that and it is the o2 sensor from what i have been told but i was also told that if your cat converters are clogged or you have them cut out all together that it will set that off as well

i am just dealing with my check engine light because i dont have cat converters