Secondary O2 sensors, tuned, still needed?

typedRew

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Feb 25, 2019
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On my last car, even though the 2nd cat codes were turned 'off' I needed the o2 sensor installed and plugged in to maintain proper fuel trims, not only did it need to be there but it could not be in a 'extended o2 tricker' housing. Had to be in the exhaust flow directly. I would get +12%-+18% trims when in the 2 inch extended housing in my aftermarket mid pipe. Chopped that off, put it in the pipe flush and trims immediately went back to normal(0 to +4%ish depending on fuel quality on 93).

That being said, I am throwing a post cat o2 code on my 09 335 and am planning to tune this week. I expected the light to go off and all to be well, because I have been told before that N54 does not care about the post-cat sensors for fuel trims.

When i install downpipes in a month or so I was going to replace my primary O2s and leave the rears as they were or just remove them from the car entirely. However, i've recently been told they are crucial for fuel trim adjustment similar to my old car and am now planning on buying four new sensors for my downpipe job and I want to know if thats something I should be doing or am I wasting money and effort?

I know a lot of guys in here have a much more intimate understanding of our DME so I wanted to pose the question.

Thanks for any help. @RSL is who brought this my attention
 
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fmorelli

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@jyamona remember this was discussed (by you) some time back - that the N54 gen forward cars are using a Bosch algorithm which takes the post cat sensor information to compensate for aging on the primary 02 sensors ... I thought you had made a comment about possibly coding this out so we could go without post-cat 02's ... or in my case selfishly, I have post cat 02s behind high flow mid cats ... but they are on a vibrant j-bung and certainly not quite in the flow as a regularly mounted sensor.

Filippo
 
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dpaul

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Jan 8, 2019
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Very interested in learning more about this topic. It is not clear to me how the activity of the post-cat sensor would be reflected in calibration of pre-cat sensors. The assertion above seems to be that the DME has an algorithm which uses post cat information to compensate for aging of the primary sensors. Sensor aging as I understand it is a pretty slow process while posters seeing trim adjustments report them as occurring relatively rapidly.

Also, I cannot wrap my head around the idea of using post-cat values as a standard to change calibration of pre-cat sensors - would post-cat values also change over time, if not from sensor aging then by aging (or removal) of the catalytic converter itself.
 

bahn

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The post-cat sensors are used to calibrate a 1.0 lambda for the widebands. The narrow bands are extremely accurate around stoich (1.0 lambda).
 
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dpaul

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Jan 8, 2019
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Thanks - that makes sense and it's the first time I've had this issue clearly explained.
 

dpaul

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Jan 8, 2019
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2009 E90 335xi, 2011 E93 M3
Thinking just a little more about this - it makes perfect sense that an NB sensor, which has a greater proportional change in output voltage around stoich than a WB sensor, would be a good way to calibrate the WB to stoich.

However, the assertion that "post cat sensor information [is used] to compensate for aging on the primary 02 sensors" does not seem right. Aging does not seem to be the issue - accurate determination of stoich is the issue. It's just harder to pin stoich down to an exact output in a WB so some sort of calibration is needed.

Does the above make sense to you? I apologize for my ignorance
 

typedRew

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Feb 25, 2019
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Has anything changed with this requirement?

seen lots of talk of tuners just getting rid of the rear sensors for people and lots of agreement. Just curious if this is still holding true and they are hurting their cars by doing this

@jyamona