code problems

i keep getting code 0430 showing up in a 1997 ford taurus. it says catalyst system efficiency below threshold bank2 . the light goes out on it's own when the weather warms up or you drive alot. it has been cold and the car has idled a lot. is this the catalytic converter plugging up or is it the 02 sensor beginning to fail?

Do the lest expensive/easy thing first and replace the o2 sensor.