Subaru Legacy P0420 code

P0420 - Catalyst System Efficiency Below Threshold (Bank 1).

Is there any way to fix this other than replacing the engine? Just going from observation I would guess it is caused by excessive wear of rings and valve guides, etc allowing the cat to get contaminated by oil. But, although I don't have any other ideas what could do it, I am curious to see if anyone else has any opinion.

The car is a 96 legacy with about 450k miles, at around 325k it did this for the first time and we replaced the cat (with a muffler shop one, oem is $2k). After just a few months it set the same code again. Then, we replaced the engine with one with 175k miles, and the code didnt come back again. After a few years, the crank pulley came loose and ruined the engine, so we replaced it with another one, but this one had 230k on it (low mileage ej22s arent easy to find anymore), and the code came right back again. If you clear the code it stays off for about 50 miles, then comes right back again. The car does not smoke or consume excessive amounts of oil, but I am not sure what else could be the cause.

That can also be a bad O2 sensor (downstream), if I recall correctly.

Man, I should have this one memorized by now. It's looking at, among other things, relative rate of fluctuations before and after the cat. I want to say it's actually the upstream one that's more critical because it's looking for the upstream to fluctuate with small adjustments to the mixture, while the downstream should be more constant once the cat's done that catalytic reaction thing.

Anyhow, sorry, I wouldn't place much emphasis on my recollection. I would look into some other details of the code and what causes it. This looks like it might cover some of the bigger picture.

Catalyst efficiency codes are most often exactly that- catalyst efficiency. Aftermarket cats are generally not near the quality of the OE ones, which is why they are cheaper.

Do you live in an inspection state? There are various solutions if you don't...

On Nissan's this is almost always just a bad cat. If the ECU doesn't see a change between the front and rear O2 sensors, then it assumes (correctly, usually) that the catalytic converter isn't doing its job. A bad O2 sensor could also make the ECU think the cat is bad, just like Ransom said. Also, like Streetwiseguy said, aftermarket cats are usually sub-par compared to OEM ones. If the engine is burning oil or dumping excessive fuel into the cats, their lifespan will obviously be much shorter.

Id imagine a decent source for an oem cat would be subie forums where pplhave ditched the stock ones. Prob can be had for a song.
As seth said, on most jap cars its usually the cat

The first time this happened changing the oxygen sensors didn't help, although it could be something different this time. Are there any other sensors on the engine that could cause this if they were failing?

I live in California, so thats about as bad as it gets for emissions related stuff. It seems really annoying that a failed cat on an OBD2 car pretty much means the car is junk unless its still fairly new. My friends mom has an Audi A6 that still runs and drivers perfectly fine, but has the same code for the cats, so once it won't go long enough without setting a code to still pass emissions testing, it will have to be parted or scrapped because new cats for that are nearly $5k. If the goal is to decrease the useful lifespan of cars (which I would assume it is), I guess thats a good way to do it.

I guess on the Subaru all I can really do is try to find an OEM cat thats still good, or get a cheap aftermarket one then just swap it on for a few days every 2 years, get the emissions test done, then put the other one back on.

Is it bad enough that it won't pass the actual sniffer test? (assuming there is no code).

If so, do what I did and hope they don't look at that downstream O2 sensor too close! I'm in a non-inspection state and until I fix the leaking valve stem seals, I am using two spark plug non-foulers to trick the ECU into thinking the cat is ok.

There is a code, so automatic fail. The last time it stayed off long enough to clear the code, drive it enough for the emissions monitors to reset and pass the test. Otherwise it wouldn't have been registrable anymore.

P0420 means bad cat 90% of the time the rest is the downstream O2 sensor. I can cause it on my modified (supercharged) Focus. All I need to do is floor it in 4th or 5th gear at low speeds. Particularly if is damp or wet weather.

Once I clear the code it will not come back until I create the situation again. Last time was almost a year ago but I had done it 3 times in 6 months before that.

My kids car is in right now with the same problem. 2001 cavalier. Codes popped after we just had the coils and plugs and ignition module replaced. Mech said its possible that the miss it had caused the converter to load up with unburned fuel thus making it go bad. Plausible? I don't know but damn are they expensive

Ok, first, don't panic. :)

P0420 means that the downstream sensor isn't showing that the catalyst is doing its job. It is comparing the difference in readings between the upstream (#1) and downstream (#2) on that bank and for some reason it is saying that the catalyst isn't doing its job.

9 times out of ten that means that the platinum in the converter is spent and no longer converting the proper molecules. That means you need a new converter.

With any 042x code, my first thing is to replace the downstream O2 sensor. There is a slim chance that it is falsely giving that code if its not doing its job.

Can you get the signal from the ds o2s? Alot of the time when a senaor goes bad it gives off a screwy reading like .05v or, say, -55 deg for an ect.
Either way the o2s will be cheap and easy

Spark plug non-fouler in between the rear O2 sensor and it's bung. You either need a deep reach 18mm non-fouler or you can double up 2 non-deep non-foulers by drilling out the one that the O2 sensor will plug into.

Pulling the O2 sensor out of the exhaust stream will make the readings different enough not to trip the code.

Now, if they get under the car to check for such shenanigans, then maybe that won't work. You could always use engine enamel and spray the non-foulers and the cat black to hide it.