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post #1 of 12 (permalink) Old 03-23-2014, 12:21 AM Thread Starter
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O2 Sensor Spacer Questions

I've read, never tried, about the O2 sensor spacer trick to fool the ECU into thinking the cat is working properly. I have a Magnaflow system on my car with a high flow cat and I have a P0420 code. My first question is has anyone done this trick and not experienced any problems afterwards? And my second is how will this affect my car during the state mandated emission testing to register my car for new tags? Thanks in advance.
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post #2 of 12 (permalink) Old 03-23-2014, 05:30 AM
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I have had this code before on a CA emissions sentra and it can become a pain. To pass emissions testing, no cel should be illuminated or automatic fail. They also test your evap system by drive cycles, we do that at our work.

To trick the o2 after cat sensor, you need to have it detect CO and O2 based upon voltage readings. If you were to trick the PCM, you would need to get a spak plug anti fouler, then possibly rethread it to fit the o2 and give it just enough to read correct 02 flow. Your code is telling the PCM that your cat converter is basically "toast". Another method I have heard from on here is to get a pcm tune performed to ignore the rear o2. I strongly suggest you just replace the cat and rear o2 together. The 2 alternatives I mentioned to trick it is nothing more than a "band aid" to a deep cut wound. They may come back or cause future issues. From a tech perspective, replace the cat and rear o2 sensor and have the pcm reset of codes.

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post #3 of 12 (permalink) Old 03-23-2014, 07:23 AM Thread Starter
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I understand that the best option is to replace the cat in the least, but I have a full exhaust system and at the time I'm unsure if I can replace the cat or possibly weld another in. If any case I'll have to get another high flow cat and it'll probably not fix the issue. My only concern until I can get it properly fixed, is if I do this, I've heard that on other cars, after doing this mod the car experiences a rougher idle and decreased mileage. Will this also be the case with the cobalt? And if I do this mod and it fixes my problem, if I were to go in for inspection, will they check and find the adapter, causing more problems?
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post #4 of 12 (permalink) Old 03-23-2014, 07:40 AM
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I've got a 200 cell high flow cat and it used to code quite often. I replaced the downstream O2 sensor with an OEM ACDelco one and it hasn't coded since. Do you have the stock O2 sensors?

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post #5 of 12 (permalink) Old 03-23-2014, 07:51 AM Thread Starter
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Yeah I believe I do. I bought the car with the light on and with the exhaust.

---------- Post added at 07:51 AM ---------- Previous post was at 07:48 AM ----------

Fact is that I don't know if it even has a cat, but I do know its a full Magnaflow system.
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post #6 of 12 (permalink) Old 03-23-2014, 08:40 AM
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I see. If the last owner removed the cat without tuning, that CEL will be constant. You'll lose power, ruin your fuel economy and fail an emissions test. Can you jack up the car and have a look or get in contact with the previous owner?

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post #7 of 12 (permalink) Old 03-23-2014, 01:52 PM Thread Starter
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Yes I've been up under the car and I noticed a very small "cat" looking item and what appears to be a resonator. And I can't get ahold of the previous owner.
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post #8 of 12 (permalink) Old 03-23-2014, 02:00 PM
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The cat will be on your downpipe and have an O2 sensor following just downstream of it (upstream O2 will be on your header/exhaust manifold). After which, there will be a flange where your catback will start. The catback will hold the resonator and muffler.


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Last edited by Plain Jane; 03-23-2014 at 02:03 PM.
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post #9 of 12 (permalink) Old 03-23-2014, 03:23 PM Thread Starter
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That is correct, but like I stated before, the one on mine is smaller.
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post #10 of 12 (permalink) Old 03-23-2014, 05:10 PM
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That's a 200 cell. Doesn't get much smaller then that I don't think.

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