P0420 - I need a CA Legal CATCON!! ...Right?

[etnpnys]

Hi all, first post here. Registered just for this question - please follow along and feel free to flog me if this is too lengthy!

I have a 2005 1ZZ-FE purchased in May of 2004 in southern California and I commute approx 110 miles every single day. I installed DC Sports headers around 60k miles, and I now have almost 210k on the car. It's been throwing P0420 for maybe 40k miles, so I doubt the headers are causing the issue - I just provide that info in case you guys know something I don't (and I hope you do, that's why I'm here!!). The car seriously still runs just like it did when I bought it - never any leaks or anything.

So anyway, I don't know what the normal behavior is of a P0420; I just clear it and it comes back in a few days. In fact, I cleared the code and it passed smog because the emissions were fine and the code didn't come back on during the test. But I'm trying to get the car ready to sell it so I'm trying to work through some of the little kinks; you know: new cabin filter, adjusting the e-brake, cleaning the interior, flushing the trans & radiator, etc. Resolving this P0420 is becoming the bane of my existence.

I have tried to take all of the more cost-efficient ways to resolve this issue:

• I replaced the second catalytic converter - it still threw the code a few days later.
• I ordered replacement oxygen sensors - this was a nightmare of incompatible parts but I currently have a new Bosch 15733 in the second location and I have the stock one in the front location in the headers.

So I called the dealership and I was like "Fine. Just tell me how much to replace the front catcon." The guy said thir.teen.hun.dred.dollars. ...Wait... Seriously? He said he can only sell the entire part from the manifold back to the muffler.

I can't find a CA approved front cat anywhere, and $1300 to replace my entire exhaust system might be reasonable but it's completely overkill!! I just need the cat!!

At this point, what do you seasoned vets recommend I do?

• Replace the front O2 sensor again with one that works? I hear that the front one usually isn't the problem...
• Bite the bullet and pay the dealership a fifth of what I can hopefully sell the car for?
• Pay an exhaust shop a little less money to weld in something that they think will work? And which part am I looking for?

Thanx in advance from someone who truly needs the help!

[fishexpo101]

I wouldn't bother with the catalytic converter, unless you KNOW that is faulty. With the mileage you have on the car, it is possible, but I would hold that as a last resort fix. I've heard of people putting in aftermarket cats and still getting them to pass Cali smog, but that is not 100% the case. If you want to do it right, do it legally, you'll have to pony up the $1300 to get the OEM cat. Might get lucky and see if someone with a low mileage later 1st gen Matrix has one they want to get rid of.

The P0420 code is generally thrown from the second O2 sensor. The upstream sensor is an AFR sensor, not a regular O2 sensor. The downstream one is a more conventional type. Get ride of the Bosch one and put in a Denso one - people have been known to get inconsistent results with aftermarket parts, especially when dealing with those sensors. The ECM is pretty picky about those sensors.

Most of your issue is from the header - it exhaust pulses are different than the OEM exhaust, so there will be difference in exhaust pulse timing. This will generally cause the downstream O2 sensor to misread the exhaust mix. Emissions-wise, the car is likely fine, just that the exhaust mix immediately sampled by the downstream O2 sensor doesn't show the behavior that it is expecting.

Three ways to address this:

Remove the header and put the OEM back in - least painful way, if you still have the manifold, PITA if you don't.

Do the sparkplug defouler mod to "space" the downstream O2 sensor further away from the exhaust stream to change its sampling (note may fail visual inspection).

Keep the header and see if a new catalytic converter will do the trick - unfortunately, there is a very good chance that it won't help, as the sensor is just not in the right place to read the mix with the header installed. This should be verified with a sniffer test - if the numbers for CO, HC, and trace comes back clean - the cat is fine.

[etnpnys]

Originally Posted by fishexpo101

Remove the header and put the OEM back in - least painful way, if you still have the manifold, PITA if you don't.

I have absolutely no idea why I didn't think of this already. It's exactly the reason I kept the stock manifold. This is probably the next thing I try, and it will probably only take me about a half an hour to do it myself.

Originally Posted by fishexpo101

Keep the header and see if a new catalytic converter will do the trick - unfortunately, there is a very good chance that it won't help, as the sensor is just not in the right place to read the mix with the header installed. This should be verified with a sniffer test - if the numbers for CO, HC, and trace comes back clean - the cat is fine.

The sensor is in the same spot with the headers. That's what the headers were designed for, was to still pass smog in CA - and they did...

I'll swap out the headers for the stock manifold again and if you don't hear from me again, it solved the problem. Else, I'll be asking for more guidance!