This is a 2000 4runner V6 manual with CA emissions. I know the code is for catalyst efficiency, but I am curious if there is any way to verify the exact issue. The code only shows up when driving at higher altitude, which is where it mostly gets driven. I have checked the oxygen sensor voltage and the upstream one reads normally, and the downstream one reads from the upper end of normal to excessively rich.
I'm guessing this means the cats need replacement, but I'm wondering how to determine why they failed in the first place.
The truck has about 270k miles on it and burns a quart of oil between changes, so I am wondering if the real issue is that the engine is worn out. New cats would cost more than a used JDM engine (and nearly as much as a rebuilt crate motor), so unless it was just for a quick fix to sell it changing the engine would make more sense if that's the case.
Is there anything else worth checking to make sure that's it's not a simpler issue? Ignoring the check engine light is an option too, although it makes it hard to tell if there is some other issue too, and it drops the gas mileage significantly when the light is on.
