02 Sienna P0420, odd question

I've done the searches but I still have questions.
Could someone please clear this up for me. My 02 Sienna, 150K miles, has a P0420 code. The check engine light would come on if I filled up at my regular station. I'd clear it and fill up elsewhere. After a month the check light now remains on always w/ the P0420 code.

So, I've just cleared the codes again and drove 10 or so miles with the check engine light remaining off. I have only the one code P0420. Does this mean that I need to change both cats b/c the engine light didn't come back on immediately? I was under the impression that a bad sensor would trigger a code right away. Since I do have 150K on the van should I just change both cats and all three sensors?

Any help would be greatly appreciated.
Jim





It was the computer.