Verified answer

If the check engine light code is specifically "Catalyst Efficiency Below Threshold" then you more than likely need to replace the catalytic converter, and there not cheap usually. There is a way to save a chunk of money by buying a universal one that list for your car at Advance auto, Autozone etc. and take it to an exhaust shop and have it welded on. Usually the direct fit, exact replacement for your car could be 2-3xs higher in price then the universal one that does the same job..

Even at 218K miles your 6 year old Scion probably does not have a bad catalytic converter. The vast majority of P0420 and P0430 codes come from feeding the converter indigestible exhaust.

The O2 sensor is a good place to start. They have a life expectancy around 100K miles, compared to the converter's life expectancy of 20 years. When they get old they get sluggish, sending ever-lengthening periods of rich and lean exhaust to the converter. At some point the converter can not absorb enough O2 during the lean periods to oxidize the CO during the rich periods and P0420 pops up. Just to complicate things I believe your upstream sensor is a wideband sensor (five or more leads) but the principle is the same. I think it is very likely it will hold the light off at least for a while longer, hopefully for many years.

The other major cause of P0420 codes is intake or exhaust leaks, but at 218K miles the sensor has had a good run and replacing it is a good place to start.