Replaced Cat, but P0420 code

Old thread: http://www.aveoforum.com/forum/f108/...nverter-15728/

My catalytic converter was destroyed (in pieces), so I replaced it with a 3rd party 49-state cat. I live in California, but did not think this would be an issue. However, now the car has a P0420 code.

It idles fine, starts fine, and MPG seems to be normal. However, when I tested it on the freeway at high RPM's, it seems to have a gas like smell.

Getting a $1,500+ California cat is not an option ATM, but I have been told by others that replacing the oxygen sensors could fix the CEL, but I've also heard that a standalone P0420 code is almost always the cat.

Would replacing the oxygen sensors be a waste of money, or could it possibly help?

with the O2 Sensors costing up to $338 at the Dealer and the Bosch aftermarket ones at Oreilly's costing nearly $200, I think its a Waste of money.

Rock Auto had both the downstream and upstream sensors for about $100~ total. Not sure what else to try at this point.

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Originally Posted by 2010AveoLT

with the O2 Sensors costing up to $338 at the Dealer and the Bosch aftermarket ones at Oreilly's costing nearly $200, I think its a Waste of money.

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I didn't check to confirm that Oreilly is $200 for a 2008 upstream, but it's way high if thats the case. Rockauto has one for around $90 + shipping. And they also are selling a Walker upstream for $39 + shipping. I have NO idea why there is such a huge price differential on these sensors, but I would try hanging one of those Walker upstream on if it were mine - not too much to lose at that price!

Edit - just noticed you posted again right before I did. Most folks generally try just the upstream O2 first, since that's the only one which normally sets that code.

I'll try that first, thanks. Interesting that one is the one that sets it, being upstream of the cat I mean.

similar issue 06 aveo LT started having P0420 code so i replace th upstream O2 sensor still get the code tried all the additives still have the 420 code replace the down stream 02 sensor still get the code so i bite the bullet and order a new cat only $100 bolt it in in (came with gasket/seals) guess what still get the code i have a cheap code reader, is there any way to tell what is really wrong without putting out too much $$$$ ?

I got a new cat from rock auto for about 190. You will need to order seals with it.

Now I am no expert. But based on my experience, it is often just the secondary o2, and not the cat.

If the cat was inoperable, causing this code, it would either be blown out, or clogged. I have never seen a cat fail in any other way. A clogged cat can do a lot of things, it can blow out, glow red, etc. But most often it will create some kind of adverse running condition. I've actually seen engines bend valves from clogged cats, it was amazing. A clogged cat should read low on a live data stream. But if you suspect a clogged cat, and its running terribly. Take the primary o2 out, if it runs better. Your cat is actually clogged. If it runs the same or worse, its not clogged.

A blown cat will do one or two things. It could run "better" because there is less exhaust restriction, louder, and rattling is possible too. It could send the catalyst downstream and filter into another part of the system. But the two things that are noticeable of a blown cat is raspy sound and smell. This is harder to diagnose, without a live data scanner you kind of have to pull the cat and look. The rusted bolts are a pita, and the expense and trouble it can be will be more expensive than just throwing in a downstream o2 to see if your ok.

Therefore, if you have this code, easiest repair is put a o2 on it and see if it goes away. Be wary of cheap, wire in o2s. NTK is ok, or a bosch plug in style are worth the few extra bucks for knowing the factory intended it to be in your car with the volt signal sweep. If the o2 does not fix it, then it is time to pull the cat.

Definitely check/swap the O2 sensor, you already have a new aftermarket cat, so assuming it's in good shape I would rule it out. I too live in CA and was lucky enough an aftermarket 3-way cat took care of the P0420. Do you know by chance if your aftermarket cat is a 2 way or 3 way cat? The one I am running is a 3 way, (platinum/palladium/rhodium) and im not sure if that would make a difference enough to throw a P0420 but just something to consider, especially if a new downstream sensor is not your culprit. As I recall when doing the ECM swap on our LT, and swapping the cat on my LS, is the ECM is programmed for a 3-way cat, whether or not it can detect this im not sure, so it may be a moot point.

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Originally Posted by petrified.rabbit

Now I am no expert. But based on my experience, it is often just the secondary o2, and not the cat.

If the cat was inoperable, causing this code, it would either be blown out, or clogged. I have never seen a cat fail in any other way.

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Another way this type of “under the header” CAT can fail is by the bolts. I had to remove an original Aveo CAT to replace the lower pipe which failed. The pipe with the flex tube under the engine was rusted through. All 3 lower bolts were beyond hope. What used to be a 10mm bolt was about 6mm and the nut was totally rounded off. All from rust; even before putting on a wrench. The top bolts on the CAT connecting to the header did not fare well during removal either. I had to use an impact wrench w/ a 6-point to get them loose. All 6 bolts on the original CAT don’t look replaceable either. I took the whole lower pipe and CAT off as a unit.

I replaced the CAT AND lower pipe. I used the original lower O2 sensor. The new CAT used 6 3/8 galvanized bolts. Unlike OEM, the bolts were not part of the CAT.

However, I had to clear a P0420 code one time since the new CAT was installed about 4 months ago. That one time clear was about a month after install.

chuck

ps: The car is in central VA now, but this 05 Aveo spent it's first 10 years in Pittsburgh where rust is a problem.