2000 4runner P0420 code troubleshooting?

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.  

I think SOP is to replace the o2 sensors and see if the code comes back. I don't think 1 quart between changes (5k?) is enough to hurt a cat.

I'm sorry to be the bearer of bad news, but 0420 is almost always a spent catalyst, and 4runners were known for having that problem.  I think you're incredibly lucky you got 270k out of them.  Lots of 4runners had to have cats replaced at 100-150k

Put it this way, in my 7 years running shops, a P0420 was never fixed with O2 sensors, and it was always fixed with a new cat.

Here's hoping you find a cheaper solution, but I predict it's actually the cats.

I'm pretty sure that it's the cats also, I have found many forum posts about the same thing.  If just the cats would fix it it's probably worth it, but if the engine is worn enough to kill them again in a couple years then that might be a different story.

I know what you mean.

There are three failure modes;  broken honeycomb which blocks flow and you'd notice it because it couldn't rev past 1500, clogged honeycomb from excessive HC like oil or rich mixture, or just plain ran outta catalyst magic.  Usually an 0420 is the last one.

Fun fact, you can diagnose the first two by pulling the upstream O2s and see if it revs higher when you give the exhaust somewhere to go.... but that doesn't sound like your issue.

In reply to Tk8398 :

Since yours is California emission, are you still living in California?  If yes, California only allows for the installation of Genuine Factory, Toyota Brand Cats (aka: expensive.)  Therefore, if you do need cats I recommend a vacation, out of state, for some aftermarket unit installation.  The price saving will pay for the vacation.  

You can try replacing the O2 sensors, but as others have said, the issue's likely the cat itself.

If you're not going to fix it, can you sell it somewhere nearby where the emissions issue wouldn't be a deal-breaker?

In reply to dj06482 (Forum Supporter) :

In California, a 2000 4Runner will go to the Mexican market so quickly that emission wont matter on price.

John Welsh said:

In reply to Tk8398 :

Since yours is California emission, are you still living in California?  If yes, California only allows for the installation of Genuine Factory, Toyota Brand Cats (aka: expensive.)  Therefore, if you do need cats I recommend a vacation, out of state, for some aftermarket unit installation.  The price saving will pay for the vacation.  

Actually, they have to be CARB EO, but they don't have to be Toyota parts.  Rock Auto shows 13 options for his 4runner that are CA-certified and starting at $218

It looks like the correct ones (there are two between the upstream and downstream sensors) plus both oxygen sensors would be about $1100 from Rock Auto for the correct ones.  Definitely cheaper than the factory stuff, although I'm not sure how the price difference relates to how long they would last or not.