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Downstream O2 Sensors


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8 hours ago, Darryl@pcmtec said:

And you can also turn the rear O2 sensor off by setting auF0011 to 0:


Do not change auF0010 to 1 on a V8. Not sure why Ford does this but it uses the number of O2 sensors on a V8 to run certain V8 code.

Excellent - works

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If you are curious have a read of the various patents Ford have filed over the years regarding downstream O2 sensors. They go into quite a bit of detail.


As Darryl said above the main use of the rear o2 sensor is to "age" the front sensor and measure the voltage shift over time which can then be used to correct for an old sensor which would otherwise need to be replaced. This system completely falls apart when you change the catalytic converter efficiency (eg high flow cat or huge exhaust) so you will not be going backwards by disabling it if you are no longer running the factory exhaust and cat. Chances are your NOx and CO are completely out the window from the factory specs once you do any modifications.

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12 minutes ago, Milanski said:

Ford spend so much time and money to work out if a $100 part is ageing?  Better of spending that money making something else work better.  Just put it down to a replace at 100k in warranty book.  Better still put in a wideband!

Think about it. 2 programmers for 1 year is maybe 250k

How many F150s are sold each year? 1.1 million. How many other Fords that use this software? Maybe 4 million?

How long does an O2 sensor last without the downstream sensor aging it? Lets say 2 years, how long does it last with the aging software? Maybe 5 years?

$100 x 4 million = $400 million in replacement O2 sensors.

Return on investment? Absolute no brainer. It isn't even a question of you can spend the money on something else, with that kind of ROI you could just borrow the money for the programmers up front and pay interest on it and still come out in front. It is cumulative as well, the FAOSC software was written in the early 2000s and still used to day, its probably saved them over a billion dollars since inception.

They don't just do this with O2 sensors they do it with everything. They have a complicated airflow model that models the cooling affect of the fans on the engine in modern vehicles, this might take someone 6 months to model in tensorflow, then a programmer a few months to program it. But if the result is the fan turns on 10% less often, this means they can change the service interval for replacing the fan by maybe 10%, how much does this save them? Even if its a $5 part it is very easy to see the economics in it.

This might also help explain why the software in a modern Ford is so damn complicated. There is usually a good reason behind everything you see, and that "good" reason is more dollars for the bean counters.

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There are plenty of examples where cars run on ONE O2 sensor - i.e. my toyota has one and seems to go just fine.  I do agree with ROI and getting 12 years out of my sensors is pretty damn good.  Also having worked with hazardous areas for a decade, no matter what code is put into a ECU - it will still be an approximation - you need to calibrate the sensors at regular intervals.  This being said, its a good point - time to replace my O2 sensors.  

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