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Post catalytic O2 sensor

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10 Dec 2019 10:20 #35697 by Aqeelalobaidi1980
Post catalytic O2 sensor was created by Aqeelalobaidi1980
Hello mr. Danner
How could the post catalytic O2 sensor effect the fuel trim in some cars?
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26 Dec 2019 17:10 #36120 by Desmond6004
Replied by Desmond6004 on topic Post catalytic O2 sensor
Someone will give a better answer but some vehicles will use the post catalytic sensor occasionally to adjust fuel trims when it sees readings that suggest that the cat can't handle the amount of fuel the normal trims is applying.

Getting involved in discussions because I have a lot to learn still.
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15 Jan 2020 21:31 #36659 by Tutti57
Replied by Tutti57 on topic Re:Post catalytic O2 sensor
I believe I remember Tyler doing some experiments with the rear o2 on a vehicle?



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16 Jan 2020 14:38 #36667 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Post catalytic O2 sensor
ISTR Mazda use it on some models.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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18 Jan 2020 12:30 #36711 by Tyler
Replied by Tyler on topic Post catalytic O2 sensor
The answer is, "It depends." :silly:

This thread may be worthwhile reading for you:

www.scannerdanner.com/forum/diagnostic-t...periments.html#26973

My general rule is, if the vehicle is equipped with an air/fuel ratio sensor, is has downstream fuel control. If it has an garden variety upstream O2 sensor, it may or may not have downstream fuel control.

How much control will depend on the specific year/make/model. For example, many Ford models I see have downstream fuel control, but it's very limited in its amount of control. 2% +/-, usually.
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Today 21:52 #43290 by Flatrater
Replied by Flatrater on topic Post catalytic O2 sensor
Upstream sensors and catalysts degrade over time with use. The downstream sensor is used (among other things) to adjust for the sensor and catalysts changes over time. For some years now, OBD2 systems are required to monitor the ability of the downstream sensor to perform this function.

It is notable that "fuel control" as described by just about everyone isn't true and hasn't been for more than 25 years. 30+ years ago there were numerous white papers and patents discussing the problems with "reactive" fuel control and its inability to fully utilize the catalyst.

We teach and use what we do because these (trim etc) are the tools given to us. I teach that it is critical to deal with trim numbers under steady state conditions (at the minimum)

Generic OBD gives us ST and LT trim control values. Yet, OEs don't necessarily "do" fuel control that way. I've been told that they are not happy with having to generate this "conversion" as it isn't necessarily correct. But for the most part it is accurate enough if we stay away from transitional values and deal with DTCs instead of trying to outsmart the onboard system.

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