2004 gts P0421

Hello Guys,

Thank you so much for the help on my previous post. I did have the wrong cylinder order. Replaced the coil that was coming up on the OBD2 and it fixed it. Car feels great!

But a day later my check engine light came back! Haha of course.

P0421 - Warm Up Catalyst Below Threshold Bank 1

I did some research already. Reset the code and it came back 2 days later. From what I can find, there is a lot of talk of the upper catalytic converter and this code. I hear that since I had been running on a mis-firing engine for a few weeks, this could have sped up the degradation of my cat. (i have around 110k miles on my car) I was told that it shouldn't really affect my driving, but I do live in CA so when i register my car Ill need to have it fixed.

Does this sound about right? Or are there other things I should try first...IE 02 sensor and whatnot. Im not really sure how to test it.

Thanks again!
Jason

think I am going to try this first

http://www.club3g.com/forum/performa...t-5-bucks.html

im running one of those spacers on each bank, 15k+ miles no problems

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Originally Posted by javoigt

think I am going to try this first

http://www.club3g.com/forum/performa...t-5-bucks.html

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That lil trick is only if your running headers or a test pipe you have another issue that has to be addressed start with replacing the down stream o2 sensor the sensor after the cat...

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Originally Posted by deeznutz

That lil trick is only if your running headers or a test pipe you have another issue that has to be addressed start with replacing the down stream o2 sensor the sensor after the cat...

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I heard that if this code wouldn't imply that the o2 sensor was bad, it would imply that the cat was bad...or not filtering enough. I guess it would make sense if the o2 sensor was giving a bad reading. Am I wrong?

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Originally Posted by Its Reu

im running one of those spacers on each bank, 15k+ miles no problems

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Was this with standard exhaust? My setup is all stock. I'm leaning towards the one after the cat would be the one to put the spacer on.

Also, before you did this did you test your 02 sensors to see what they were reading?

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Originally Posted by javoigt

I heard that if this code wouldn't imply that the o2 sensor was bad, it would imply that the cat was bad...or not filtering enough. I guess it would make sense if the o2 sensor was giving a bad reading. Am I wrong?

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True, the PCM gets its info about the catalyst from the o2 sensors, so if one of them was going bad then you'd get this code too, which is why people say to start with the o2 sensor, and if you get the code again you know it's time for a new cat. And since it's good practice to replace the sensors when you replace the cat anyway, you didn't really waste money on a new o2 sensor.

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Originally Posted by wetamup2k3g

True, the PCM gets its info about the catalyst from the o2 sensors, so if one of them was going bad then you'd get this code too, which is why people say to start with the o2 sensor, and if you get the code again you know it's time for a new cat. And since it's good practice to replace the sensors when you replace the cat anyway, you didn't really waste money on a new o2 sensor.

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Thanks! I think I may try the trick first. Might as well for 5 bucks. If its a no go then Ill go down the line...o2 sensor...cat converter.

The other way to know for sure what is going on would be to get a scan tool hooked up and watch the voltage signal on the downstream O2 sensor. This will also tell you for sure if it is a bad catalytic converter or a bad O2 sensor.

The spark plug fouler trick works pretty good and it is cheap. Just make sure you get the 18mm short ones. Drill out the first one with a 1/2" drill bit and thread into the second one.


NOTE - I have found that with some of the Bosch brand O2 sensors the sensor portion that rests within the exhaust stream is a bit larger than some of the OEM sensors. This is something to keep in mind because you won't have much metal material between the inside and outside diameter and it will shear right off. Simply welding the two pieces together (heck even a couple tack welds) will strengthen it more so that it doesn't shear off when you tighten everything in.

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Originally Posted by SPD_FRK

The other way to know for sure what is going on would be to get a scan tool hooked up and watch the voltage signal on the downstream O2 sensor. This will also tell you for sure if it is a bad catalytic converter or a bad O2 sensor.

The spark plug fouler trick works pretty good and it is cheap. Just make sure you get the 18mm short ones. Drill out the first one with a 1/2" drill bit and thread into the second one.


NOTE - I have found that with some of the Bosch brand O2 sensors the sensor portion that rests within the exhaust stream is a bit larger than some of the OEM sensors. This is something to keep in mind because you won't have much metal material between the inside and outside diameter and it will shear right off. Simply welding the two pieces together (heck even a couple tack welds) will strengthen it more so that it doesn't shear off when you tighten everything in.

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Thanks, going to try the trick tonight. Will let you know how it goes!

I did the little spark plug defouler trick...So far so good...75 miles and counting. Last time the light came on after only 30...*fingers crossed*

100 miles and the light came back on :/ just to clarify...I put the spacer on the second o2 sensor seen from the front, right after the small CAT, this was the right one for this code right?

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Originally Posted by javoigt

100 miles and the light came back on :/ just to clarify...I put the spacer on the second o2 sensor seen from the front, right after the small CAT, this was the right one for this code right?

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Scratch that...I think I answered my own question, it looks like I should have a sensor close to the firewall (Bank 1) since thats where cylinder 1 is . I think I put the spacer on (Bank 2)....Im learning at least ha...