Recently I finished sorting out exhaust leaks after installing my OBX headers on my truck. The truck has been running very rich for awhile because it was basically open headers for the longest time but even after the leaks were fixed and everything is good I keep having the P0420 code come up. I've cleared it a few times and its still been coming back. Not sure if it is my oxygen sensor thats bad, the cat thats bad, or what honestly.

I should mention that when I had the exhaust done I had the pre-cat or first cat deleted (the one closest to the engine) and had basically a test pipe put in place. The rest of my exhaust consists of just a glasspack and stock diameter piping, which I never had a problem with before the headers

Thoughts on which direction to proceed?

I'm no expert but it seems the air/fuel sensor is what was removed?

If so how can the 'puter read/adjust the fuel trim?

Sounds like the bung for the A/F sensor was in the new headers, similar to my TRD headers. However, with the Cat removed I'm pretty sure you're going to need one of these for your rear O2 sensor.

http://urdusa.com/store/Electronics-...6372c7d1a5bd80

Yeah there is a bung for the A/F sensor, I'm assuming this is what it is supposed to look like?


Is it possible that that my O2 sensor is bad? I remember that URD has that product and I was looking into that as a last resort.

You can take a look at my build thread for the exhaust if you want. It's here. My Exhaust system

My TRD headers came with a bung for the A/F sensor. I'm using it for the AEM gauge. The photo you have is correct. That should be your A/F sensor.

I also have an additional bung that came with my upstream cat which I'll use for my A/F sensor. I chose to keep both of my Cats.

(EDIT: If I wasn't running an AEM A/F gauge for the SC I would only have an upstream and a downstream bung.)

The third bung, or in your case the second bung which you should have but since you went with a straight pipe I don't believe it's on there would be the sensor reporting the condition of your CAT to your computer. This is your O2 sensor on the DOWNSTREAM side of your Cat.

If you don't have it you will constantly get a 0420 code. The way some folks get around it is to "ignore" it or take the CEL bulb out. The other way to get around it for DEQ is to have the additional bung welded in the pipe by any muffler shop and install the URD option.

Yeah, the P0420 is the rear O2 sensor. If it's not connected after the cat it will throw a code.

If it is in there and still throwing a code, replace it.

your problem is the cat converter. when the PCM is tricked out by the exhaust leaks meaning high oxygen in the exhaust it then turns on the injectors to max to get the fuel trims/fuel ratio to the correct PCM programed spec.. then the converters get carboned up and the cat eff code 420 is thrown.

having a scan tool that can read the O2 sensor voltages and chart them you can see how the O2 sensors up and downstream switch from high to low. when the downstream switches like the upstream that means the converter is dead.

I took a look at your exhaust setup and it looks really good! It also cleared up a lot of things. So yes the cat that I deleted is the upstream cat. I left the downstream alone alone and the O2 sensor which is past the downstream cat alone as well.

The bung that I have on my headers is for the A/F sensor. I may get a bung welded into the pipe replacing the upstream cat for a wideband sensor, but thats later on.

I believe my cat is fine, and I may eventually replace it with a high flow cat, but I do believe that my only option for clearing the code for good is getting that URD attachment.

Thanks for all the help!

Under what conditions does your trigger the 0420? Mine will do it consistently when I travel offroad (road could still be pretty smooth) and about 5-10mph, boom everytime. I may replace the rear sensor as the front one was done already.

Exactly what happens to mine. Let me know if that works for you.

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No problem. I'd be curious to know what actually fixes it. Instead of the URD sensor simulator you can order a new O2 sensor (Denso) and see if it clears. As most have said that's usually the culprit. Good luck.