2002 Toyota Sienna P0420 Code: Do I Need to Replace Both Catalytic Converters at 150K Miles?

I've searched online but still have some confusion about this issue. My 2002 Toyota Sienna, with 150,000 miles, has been showing a P0420 diagnostic trouble code. The check engine light would come on only when I filled up at my usual gas station. After clearing the code, I'd drive to another station and the light would go off again—only for it to return after about a month and stay on permanently. I recently cleared the codes once more and drove approximately 10 miles with the check engine light remaining off. Currently, only the P0420 code is present. Does this mean I need to replace both catalytic converters? I originally thought that if a sensor or converter was faulty, the check engine light would come on immediately. Since my van has reached 150K miles, should I simply replace both catalytic converters and all three associated sensors? Any insight or advice from experienced owners or mechanics would be greatly appreciated. Jim (Note: The issue was actually resolved by replacing the oxygen sensor in the exhaust system.)