p0420 CEL - what to replace?

[daniel_gabriel]

I've got 2 of them, resulting in a 'cat efficiency below threshold' message. I know there are 2 catalytic converters on the car. I presume that because there are 2 codes, they come from the front and rear oxygen sensors. My question is, which catalytic converter to replace?
In this picture (http://www.rockauto.com/ref/Walker/D...html?1652.jpg), do I need to replace part 16089 or 16090?

Thanks in advance,
-Daniel

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[Matt Monson]

Do you have headers? If so, the headers are causing the code. It's really rare to actually have a cat go bad. I have run with an 0420 code for about 3 years now. It doesn't effect driveability at all...

[EJ1]

If you are under 80K/8 years emissions warranty, just go to a dealer and voila!... new cats. If not, you can live w/ it, get a CEL or mechanical fix. Just do a search for those. There's a slight chance that it's the rear o2, but probably not. It could be off (not necessarily bad) and cause the P0420 to be thrown.

[daniel_gabriel]

everything is completely stock, and the car is a 99 OBS with about 92K on it. I live in MD, and I have emissions testing coming up, so I need to get rid of the code before then. Hence the question. I've been told previously by a few other members that it indeed is the cat causing this code to be thrown.
Do you know if the O2 sensors are before and after the cat on the exhaust manifold, or the cat on the downpipe?

[Siper2]

The sensors bolt directly into the actual cats I think.

P0420 on a car with a high-flow cat pipe unit, and/or headers is usually a fault of the rear O2 sensor, because there's no cat there. There's a high-flow cat in the forward location, no cat in the rear. So you need one of the PDM O2 bung extension units to try and eliminate it.... but mine never worked.

On a stock car, though, my guess would be rear cat, but I suppose it could be either. Subaru CEL code log:
http://forums.nasioc.com/forums/show...1&postcount=12

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[Master2192]

Alot of times this code gets thrown when the rear O2 sensor is going out.

[daniel_gabriel]

So, in the worst case, I would need to replace both O2 sensors, and which cat? the one on the manifold, or the one on the downpipe?

[G.Subramaniam]

Rallitek sells a fix for the 420 CEL

[FalconRS]

Kay first off, the front O2 sensor is NOT there to monitor the cats. It comes before the cat, and reports the air-fuel ratio back to the ECU so the ECU can in turn adjust fuelling to keep the car running properly. You will NEVER get a cat inefficiency code from the front O2. The rear O2 is there exclusively to monitor the cats. But it will only throw ONE code if there's an inefficiency detected. It'll throw a code if one or both cats are going bad (no way to tell if it's one or both), or it will throw a code if the sensor is going bad. It can also throw a code if your cats aren't heating up to spec (mostly caused by headers) because cats use heat to initiate the catalyzing process.

Want a real easy solution? Replace the sensor first to rule that out. Second, tons of guys around here have bought high flow cat pipes or track pipes, replacing their stock cats. Buy one of their stock units and you're off to the races. There are no guarantees that a high flow unit will pass emissions (probably won't) so don't listen to the guys around here that try to talk you into it. Your emissions check is coming up, don't mess around.

EDIT: Should also add, with the age and mileage on your car, it's entirely possible that the sensor's toast, or the cats are either shot or clogged. At that mileage, it could be either or both. It's definitely one of them. Hope this helps.

[daniel_gabriel]

Thanks for your help.
So the front sensor is before the cat on the manifold, and the rear sensor is behind the cat on the downpipe?
And, in the worst case, I would have to replace the sensor and both cats, correct?
Thanks again,
-Daniel

[DreddRS25]

Originally Posted by daniel_gabriel

Thanks for your help.
So the front sensor is before the cat on the manifold, and the rear sensor is behind the cat on the downpipe?
And, in the worst case, I would have to replace the sensor and both cats, correct?
Thanks again,
-Daniel

This may help with your question. The front sensor (A/F) is right infront of the first cat, it has the black wire (one on the right). The rear 02 sensor is at the rear of the rear cat, it has the red wire. So directly, both sensors are located on the cat pipe itself.

[daniel_gabriel]

Thanks for the picture, it helps much.
So, if after I replace the sensor the code comes back, there is no way whatsoever to distinguish which cat is the one to be replaced?

[FalconRS]

Like I said, your best best is to just grab a stock cat section off somebody here who's upgraded to a high flow unit. You can probably get that cheaper than getting the dealership to troubleshoot out this one (or any other garage).

[daniel_gabriel]

Thanks for your replies. I have ordered a rear O2 sensor and will be installing it probably this weekend. We'll see how it goes and if it fixes the problem.

[Siper2]

Here's another decent comparison photo.



This is a Stromung HFC that I had in my car, just took it out and sold it last week. I'm saving up for a turbo swap, so this unit does me no good once I put the new engine in.

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