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Discussion Starter #1
Been reading up about O2's and just want to clarify if I got
this right. The O2 on the header just before the Primary cat
is for emission purposes, where as the O2 after the Primary
cat is for diagnostic purposes, correct? If this be the case
and I was to install an aftermarket header, wouldn't a check
engine light be tripped?
 

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The pre-cat O2 sensor aids in adjusting the air/fuel mixture and keeping the engine from running too lean or rich. The after-cat O2 sensor allows the ECU to verify the cat is working properly and the overall emissions of the car are within tolerance. The after-cat sensors are generally only for OBD-II applications. If you install an aftermarket header, it should have the boss for the pre-cat sensor. I'm not sure about the after-cat sensor, though. I though B14's have the cat coupled to the stock header and thus you'll have to add a new converter and have a boss welded into the exhaust downstream of the cat.
 

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From the B14 Factory Service Manual:

"This system [Mixture Ratio Feedback Control] uses a front heated oxygen sensor in the exhaust manifold to monitor if the engine operation is rich or lean. The ECM adjusts the injection pulse width according to the sensor voltage signal. ...... Rear heated oxygen sensor is located downstream of the three way catalyst. Even if the switching characteristics of the front oxygen sensor shift, the air-fuel ratio is controlled to stoichiometric by the signal from the rear heated oxygen sensor"

You won't trip an error code because headers come with a bung for installing the front oxygen sensor.

EDIT: Above message posted while I was composing this one, sorry for the redundancy. The catalytic converter in B14s is not attached to the header. The rear sensor is mounted in the back of the converter.

Lew
 

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Discussion Starter #4
lshadoff said:
From the B14 Factory Service Manual:

"This system [Mixture Ratio Feedback Control] uses a front heated oxygen sensor in the exhaust manifold to monitor if the engine operation is rich or lean. The ECM adjusts the injection pulse width according to the sensor voltage signal. ...... Rear heated oxygen sensor is located downstream of the three way catalyst. Even if the switching characteristics of the front oxygen sensor shift, the air-fuel ratio is controlled to stoichiometric by the signal from the rear heated oxygen sensor"

You won't trip an error code because headers come with a bung for installing the front oxygen sensor.

EDIT: Above message posted while I was composing this one, sorry for the redundancy. The catalytic converter in B14s is not attached to the header. The rear sensor is mounted in the back of the converter.

Lew
Little confused....the Primary cat on my car is an intregal part of the
exhaust manifold and my rear sensor is located before the secondary cat.
If I remove my stock manifold and install an aftermarket, the absence of
the primary cat will not affect what the rear O2 will read (verify)?
Also, just a note, the header I am looking into has bungs for both the front
and rear O2. Thanks.
 

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Mofo
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I think the header you are looking at does have 2 bungs, but one is for the o2 sensor, and the other one is for your egr tube.
When you install the header put the front o2 into the bung in the new header.
Now you are going to have to lengthen the wires on the rear o2, and have the exhaust shop place a bung for the rear o2 behind the under car cat.
I had the same problem on my old b14. The shop told me it would be ok just to put a bung into the down pipe of the header where the old one was, but it was reading wrong, due to me removing the cat/ exhaust manifold. So I lengthened the wires of the o2, and then had a bung placed behind the under car cat. I didn't have any problems after that.
The only problem I had with the o2 sensor in the wrong place was bad gas milage, and some stinking ass exhaust fumes that I can tell anyways.....
This is just my experience with this.....
 

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Discussion Starter #6
Nostrodomas said:
I think the header you are looking at does have 2 bungs, but one is for the o2 sensor, and the other one is for your egr tube.
When you install the header put the front o2 into the bung in the new header.
Now you are going to have to lengthen the wires on the rear o2, and have the exhaust shop place a bung for the rear o2 behind the under car cat.
I had the same problem on my old b14. The shop told me it would be ok just to put a bung into the down pipe of the header where the old one was, but it was reading wrong, due to me removing the cat/ exhaust manifold. So I lengthened the wires of the o2, and then had a bung placed behind the under car cat. I didn't have any problems after that.
Makes total sense

The only problem I had with the o2 sensor in the wrong place was bad gas milage, and some stinking ass exhaust fumes that I can tell anyways.....
This is just my experience with this.....
Seems as though this caused a richer A/F mixture. Hmmm...wonder
if an adjustable fuel pressure regulator would solve this problem.
 

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TwiztidKidd
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I only have one oxygen sensor and I think the o2 sensor would be more accurate if it was placed on the downpipe after the 4-2-1. The way it is now its got a reading on cylinder no.3 only. The factory exhaust manifold wasn't designed that way.

 

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Discussion Starter #9
Nostrodomas said:
I think the header you are looking at does have 2 bungs, but one is for the o2 sensor, and the other one is for your egr tube.
When you install the header put the front o2 into the bung in the new header.
Now you are going to have to lengthen the wires on the rear o2, and have the exhaust shop place a bung for the rear o2 behind the under car cat.
I had the same problem on my old b14. The shop told me it would be ok just to put a bung into the down pipe of the header where the old one was, but it was reading wrong, due to me removing the cat/ exhaust manifold. So I lengthened the wires of the o2, and then had a bung placed behind the under car cat. I didn't have any problems after that.
The only problem I had with the o2 sensor in the wrong place was bad gas milage, and some stinking ass exhaust fumes that I can tell anyways.....
This is just my experience with this.....
Ok, went back and re-read this. So now, let me see if I got this flying
straight. By placing the rear O2 after the secondary cat, your gas
mileage and stink demons were resolved..correct? In my head this makes
sense being the rear O2 will be reading (verifying) what the secondary
cat is doing.
 

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2018 Nissan Pathfinder SL
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BRSGXE said:
Little confused....the Primary cat on my car is an intregal part of the
exhaust manifold and my rear sensor is located before the secondary cat.
If I remove my stock manifold and install an aftermarket, the absence of
the primary cat will not affect what the rear O2 will read (verify)?
Also, just a note, the header I am looking into has bungs for both the front
and rear O2. Thanks.
Yes, you have a 97 GXE same as me with the GA1.6 and Lew has a SER with a SR2.0 and the 2.0's have a separate CAT. As I have read on the forum here you need a 2.0 CAT when you buy headers for a GA1.6
Well if the bungs are both on the header it will work regards of the CAT I guess.
Good Luck....
 

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Discussion Starter #11
IanH said:
Yes, you have a 97 GXE same as me with the GA1.6 and Lew has a SER with a SR2.0 and the 2.0's have a separate CAT. As I have read on the forum here you need a 2.0 CAT when you buy headers for a GA1.6
Well if the bungs are both on the header it will work regards of the CAT I guess.
Good Luck....
Sorry, should have been more specific on engine application. I do have
the 1.6. Now about the 2.0 cat, by any chance does this cat have an
O2 bung at the back of it? And is this why you are recommending I use
one if going with an aftermarket header? Thanks.
 

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BRSGXE said:
Sorry, should have been more specific on engine application. I do have
the 1.6. Now about the 2.0 cat, by any chance does this cat have an
O2 bung at the back of it? And is this why you are recommending I use
one if going with an aftermarket header? Thanks.
I have not done this yet so someone else would be better qualified to answer.
I can look under my daugters SER but since she is in another town it may take a week for an answer from me.
Good Luck......
 
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