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Author Topic: Post Cat o2 sensor?  (Read 677 times)

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Offline Powermizer

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Post Cat o2 sensor?
« on: September 05, 2014, 09:08:18 PM »
Does the ECM calibrate fuel trims using the post cat o2 sensor?
Didn't know if I should put this in the tuner section!!!
Sorry if this has been beat to death, I couldn't find what I was looking for with search......thanks

Offline tazz

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Re: Post Cat o2 sensor?
« Reply #1 on: September 05, 2014, 10:07:11 PM »
Only pre cat o2 sensor used by ECM to adjust lambda value.   Post cat only used for cat efficiency at least to my knowledge.
Could be wrong though.

Offline Powermizer

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Re: Post Cat o2 sensor?
« Reply #2 on: September 06, 2014, 01:42:24 AM »
I've heard both ways. I would love a for sure answer on this. These ECM's are so smart anymore!!!

Offline Kelu

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Re: Post Cat o2 sensor?
« Reply #3 on: September 06, 2014, 02:40:33 PM »
Only pre cat o2 sensor used by ECM to adjust lambda value.   Post cat only used for cat efficiency at least to my knowledge.
This is what I know too ;)
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Offline Dave@DDMworks

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Re: Post Cat o2 sensor?
« Reply #4 on: September 06, 2014, 05:17:36 PM »
Although the primary O2 sensor is the main sensor used by the ECM for fueling, the secondary O2 sensor can have an affect on the fueling. We learned this with Miller's car at the Maryland Mod Meet one year when the O2 extender on the sencondary O2 sensor failed. Basically it allowed air to be sucked in past the secondary O2 sensor while at idle, which read as very lean, basically air. What the ECM then did was after about 3 minutes if I remember, it ran the engine very rich for about 10 seconds, then ran normally. Then since it saw no response from the secondary O2 sensor, it did it again and ran the engine really rich for about 10 seconds. It did this a total of 3 times, after which it ran in kinda a fueling limp mode. I could be wrong on the times it took, etc, as it was about 4-5 years ago now, but it was pretty interesting. That being said, if the secondary O2 sensor is working normally the primary O2 sensor is used for fueling.
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Offline Powermizer

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Re: Post Cat o2 sensor?
« Reply #5 on: September 07, 2014, 12:35:43 AM »
Thanks Dave, I thought something like that may be happening. Now the real question comes.

If we have a cat less down pipe and we use a o2 extender or we change the voltage that the ECM sees, to keep the P0420 light off, does this mean our fueling map will be off or that the ECM could go into limp mode?

I have never heard of anyone stating a problem!!!   Thanks

 

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