P0420 cat code

My question is, the up stream or second cat? Truck has 275k on it! And go oem or aftermarket?

 

Primary cat thats part of the manifold.

They are pricey but OEM is the only way to go.

You can leave p0420 for as long as you want, it has no symptoms, but it will hide any new codes that come after.

Last option, is a spacer for the downstream sensors. https://www.summitracing.com/parts/rnb-42002?seid=srese1

This is a no-go in emission states/provinces, but if money is tight you can use a downstream spacer that tricks the truck into clearing the P0420, you can read up on them on 4runner pages and tacoma pages.