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Stephen H - 16 Dec 2005 06:31 GMT

My world is upside down... Just learned tonight that the post cat o2 sensor
does effect fuel control!

I'll type up the case study for you soon... Real crazy.

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Stephen W. Hansen
ASE Certified Master Automobile Technician
ASE Automobile Advanced Engine Performance
ASE Undercar Specialist

http://autorepair.about.com/cs/troubleshooting/l/bl_obd_main.htm
http://www.troublecodes.net/technical/

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TeGGeR® - 16 Dec 2005 08:25 GMT

> My world is upside down... Just learned tonight that the post cat o2
> sensor does effect fuel control!

No way!?

On all OBD-II cars, or just the way Honda does it?

> I'll type up the case study for you soon... Real crazy.

I'd love to see it. Thanks.

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TeGGeR®

The Unofficial Honda/Acura FAQ
www.tegger.com/hondafaq/

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Stephen H - 16 Dec 2005 14:44 GMT

All OBD 2 cars for sure, although there is NOTHING in print about it that
the average Joe will find in writing.
The class was long last night, got to get to work.
Chat later

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>> My world is upside down... Just learned tonight that the post cat o2
>> sensor does effect fuel control!
[quoted text clipped - 6 lines]
>
> I'd love to see it. Thanks.

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Stephen H - 23 Dec 2005 06:12 GMT

CTI Book Page 104 (CarQuest technical Institute)

….. What is the purpose of the post cat Oxygen sensor?

--Catalyst efficiency monitor?

That’s only a portion of its function

REAR FUEL TRIM

a.. The Post-cat O2S has been used since 1988 to control fuel trim (to
some extent)
a.. Toyota and Saab
b.. Used to fine-tune the A/F ratio to maximize catalyst efficiency
c.. Will also compensate for a degraded Catalyst
d.. Every manufacture today uses the rear O2S to trim fuel today

The post cat O2S is a fuel control input. Toyota and Saab have used the
post-cat O2 to control the fuel trim since 1988; Before OBD-II. This sensor
is used to fine-tune the air-fuel ratio to maximize catalyst efficiency. It
can also adjust the air fuel ratio to compensate for a degraded Catalyst. If
you were to measure the actual amount of time the post-cat O2 is used during
the running of the catalyst monitor (maybe once per trip) and compare that
to the amount of time the post-cat O2 is used fine tune the air-fuel ratio
(almost always in closed loop), you will see that this sensors major
function is fuel control. Every vehicle manufacture today uses the rear O2S
for fuel correction; even if it is undocumented.

a.. How much control over fuel trim does the rear O2S have?
a.. GM material says less than 1.0%
b.. Ford says 0.5%
c.. Toyota says 2.0%
d.. Under normal conditions
b.. We have seen up to 30% correction in fuel trim from the rear O2S in
abnormal conditions
(SNIP)

According to these manufactures, the post-cat oxygen sensor has minimal
control over fuel trim under normal operating conditions. GM material states
the rear O2S has +/-1.0% authority over fuel control. The Ford OBD-II manual
leads us to believe this sensor is of little consequences in regard to
drivability as it only has +/-0.5 fuel control authority. Toyota allows for
a whopping 2.0% +/- correction. However these claims are under normal
driving circumstances. What happens if there is a problem? We have
documented up to 30% correction in fuel trim from the rear O2S in abnormal
conditions.

(SNIP)

Rear fuel trim Experiment

The Guinea pig; 1999 Dodge Grand Caravan Sport. 3.3 Vin G. 168000 miles

We wanted to determine what, if any, input the rear O2 sensor has on fuel
trim. Many vehicles have rear fuel trim PIDs available. Some OEMs have even
documented the range in which the rear O2 can affect total fuel trim. On
this vehicle however, Chrysler specifically states that the sole function of
the rear O2 is for the catalyst monitor. Period! The rear O2 sensor has zero
input on fuel control. The idea behind this experiment is to control the
rear O2 sensor voltage to the minimum and maximum extreme of normal values
for an extended period of time and monitor short term and long term PIDs on
the scan tool. The rear O2 voltage will be controlled with a sensor
simulator.

(Snip) Steve abbreviates:

a.. Drive 1 Normal conditions
a.. B1S1 Switching 100MV and 900MV
b.. B1S2 steady at 600MV
c.. Fuel trim
a.. Shot term FT +4%-4%
b.. Long term +3%
Drive 2

The sensor is set to 120 MV, low end of the range B1S1 O2s is stuck full
rich; except for the few times the engine experienced slight misfires due to
the over-rich condition.

Short-term fuel trim has apparently reached its limit at +25%.
Long term stopped at +22%

Since STFT is fairly fixed its easy (to calculate the total fuel trim) +47%!
NO CONFERMED DTC’ WERE SET! We do have the following pending DTCs: P0172
Fuel system rich B1S1; P0170 Fuel trim fault B1S1. Notice the PCM is
ignoring the B1S1 O2S because it is attempting to optimize the catalyst.
This is the PCMs priority

(Note MPG went from 20 MPG to 15)

Drive 3

Thee sensor is set to 850 MV, the high end of the range B1S1 O2s is stuck
dead lean; Ran fine (but couldn’t climb a hill)

Short-term fuel trim has apparently reached its limit at -20%.
Long term stopped at -25%

Since STFT is fairly fixed its easy (to calculate the total fuel trim) +47%!
Once again NO CONFERMED DTC’ SET! We do have the following pending DTCs:
P0171 Fuel system lean B1S1; P0170 Fuel trim fault B1S1; P0131, B1S1 Shorted
to ground.

Note the codes do not tell your anything about the rear o2 sensor?

(Note MPG went from 20 MPG to 25)

Anyway, that sums up the first case; I had to abbreviate some stuff
(pictures and graphs and stuff) but it gives you something to think on.

There is another case about a Ford escape; perhaps someday I’ll type it up.

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TeGGeR® - 24 Dec 2005 17:18 GMT

> CTI Book Page 104 (CarQuest technical Institute)
>
[quoted text clipped - 7 lines]
>
>

<snip>

I'm just really curious why this function would be sometimes undocumented,
and why automakers would downplay the influence the secondary sensor has no
fuel trim. I can't see the motive for the minimization.

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TeGGeR®

The Unofficial Honda/Acura FAQ
www.tegger.com/hondafaq/

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Elle - 24 Dec 2005 18:08 GMT

> "Stephen H" <hansensw@hotmail.com> wrote
> > CTI Book Page 104 (CarQuest technical Institute)
[quoted text clipped - 5 lines]
>
> > REAR FUEL TRIM

> I'm just really curious why this function would be sometimes undocumented,
> and why automakers would downplay the influence the secondary sensor has no
> fuel trim. I can't see the motive for the minimization.

I don't think it's completely accurate to say the rear
oxygen sensor's purpose is "fuel control." From what Steve
appears to quote from the manufacturer, its purpose is
"optimizing catalytic converter performance /via/ fuel
control input."

Of the information from three manufacturers Steve lists, the
Toyota's rear O2 sensor appears to have the potential for
the greatest effect on fuel control, affecting trim by as
much as plus or minus 2%. Presumably, it does this to
minimize deleterious exhaust emissions. But I would also
presume that, with the rest of the engine control system,
the rear O2 sensor is simply only one part of a careful
(control system) balancing act for optimizing engine
efficiency and minimizing emissions.

One of the bottom line questions on my mind is: Under normal
operating conditions, does the rear oxygen sensor sacrifice
MPG to improve cat converter efficiency? If it does, it
surely must be such that the overall deleterious emissions
from the car are less /per mile yada/ than if the rear
oxygen sensor didn't make any adjustment. Otherwise, there's
no point in having the rear oxygen sensor have an input to
fuel control. In other words, burning more gas per mile
increases emissions, unless the cat converter efficiency
rises in turn. Or there's some other tradeoff that justifies
the (I assume) lower MPG.

I picture a sequence possibly like the following over the
life of a car:
-- cat converter catalyst yada is slowly consumed
-- cat converter efficiency goes down slowly
-- emissions rise slowly; fed government says, "Stop this."
-- rear O2 sensor fine tunes fuel trim. This lowers MPG but
raises cat converter efficiency so that the dominant effect
is lower emissions per mile.

The closer to the end of life the cat converter is, the more
the rear oxygen sensor adjusts the fuel trim. BUT that's
only a temporary situation, since the car is approaching a
highly abnormal operating condition (no working cat
converter!). So the car will end up in the shop soon enough.
ISTM that the rear O2 sensor's fuel control input is mostly
to minimize emissions near the end of life of the cat
converter.

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Stephen H - 25 Dec 2005 06:25 GMT

The instructor said several times during the class "GOD SAVE THE CAT!)
We tend to think about emissions and MPG and such, but realistically, the
cars CPU's job is to save the cat regardless of the consequences to
drivability. IF the Mil comes on' its saying "something is wrong that may
damage my cat"
The car's response is to adjust fuel trim and such to correct the numbers.
But as the test showed, the rear sensor, (that I was told was just a
monitor) (and by the way was also shown as a monitor for the cat effiency in
the ASE Test) radically screwed with the fuel trim because it didn't like
what it saw.

I believe the instructor said in starting in the 2005 year of cars, "rear
fuel trim" (a term not normally used) will start showing up on scan tools
and therefore start becoming a normal diagnostic tool.
Why it wasn't acknowledged? who knows.

>> I'm just really curious why this function would be
> sometimes undocumented,
[quoted text clipped - 7 lines]
> "optimizing catalytic converter performance /via/ fuel
> control input."

I think you on to something there... It effects fuel control but only to
assist the cat in it's job.

> Of the information from three manufacturers Steve lists, the
> Toyota's rear O2 sensor appears to have the potential for
[quoted text clipped - 9 lines]
> operating conditions, does the rear oxygen sensor sacrifice
> MPG to improve cat converter efficiency?

Yes it did in the Dodge experement.

If it does, it

> surely must be such that the overall deleterious emissions
> from the car are less /per mile yada/ than if the rear
[quoted text clipped - 22 lines]
> to minimize emissions near the end of life of the cat
> converter.

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Elle - 25 Dec 2005 16:42 GMT

> The instructor said several times during the class "GOD SAVE THE

CAT!"

> We tend to think about emissions and MPG and such, but realistically, the
> cars CPU's job is to save the cat regardless of the consequences to
> drivability. IF the Mil comes on' its saying "something is wrong that may
> damage my cat"
> The car's response is to adjust fuel trim and such to correct the numbers.

But assuming other symptoms (codes) come up that tell the
owner to get the car into the shop, this makes sense: The
catalytic converter is expensive. In abnormal conditions
that could damage the CC, having the car go into a mode that
preserves it is a good engineering choice.

So we could throw out my proposition that the fuel trim
input from the rear oxygen sensor has the net effect of
reducing emissions. It probably doesn't, because your
evidence shows MPG goes down in response to a signal from
the rear oxygen sensor in abnormal conditions. Emissions go
up, but the Cat is saved.

> But as the test showed, the rear sensor, (that I was told was just a
> monitor) (and by the way was also shown as a monitor for the cat effiency in
> the ASE Test) radically screwed with the fuel trim because it didn't like
> what it saw.

But that was in a radically abnormal operating condition,
correct? So the owner-operator of such a car would, in
theory, be hustling to a shop because of other symptoms,
ISTM.

> I believe the instructor said in starting in the 2005 year of cars, "rear
> fuel trim" (a term not normally used) will start showing up on scan tools
> and therefore start becoming a normal diagnostic tool.
> Why it wasn't acknowledged? who knows.

For now, I still think it's minimized because its
significant only in highly abnormal operating conditions.

As for "rear fuel trim" becoming a term that is more widely
used in the future: I can't say for sure, but ISTM its just
adding another diagnostic to the toolbox. Perhaps a good
one, especially for a tough diagnosis. (I really couldn't
say exactly. I'm only going by what I think your summaries
are saying. Maybe if I sat in this class, my take would be
different. Ultimately, though, I think we have to believe
that the designers of the rear O2 sensor fuel trim input
were rational people. Why adjust the fuel trim in part in
accordance with what comes /out/ the cat converter? The only
reason would be to preserve the expensive catalytic
converter, ISTM.

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Elle - 25 Dec 2005 17:13 GMT

Which CTI course are you taking again?

Just googled for "rear fuel trim" and got only one hit:
Another course offered by Carquest Tech Instititute. It's
relevant and a little interesting. Here's the description:
---
Total Fuel Trim Diagnosis
Fuel Trim is the technician's best chance to find the right
diagnostic path to solving most all drivability and engine
performance problems. This course will dispel the myths
about short and long term fuel trim and Adam will introduce
the concept of Total Fuel Trim. It is important that the
PCM's total fuel modification be taken into consideration
when diagnosing these problems. The effects of Rear Fuel
Trim found on most all vehicles today, will also be
explained. This is a must see class that will open your eyes
to the power of Total Fuel Trim diagnosis.
---
http://www.asashop.org/cars05/attendees_tech.htm

(This site lists a course with evidently a nice plug for
technicians using online discussion groups to troubleshoot,
as well.)

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Stephen H - 26 Dec 2005 06:28 GMT

Yes, this sounds like the class I went to.
Am scheduled for 5 more next year, using "Volumemetritc efficiency" to
diagnose problems. And I just deleted my calculator

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>> I believe the instructor said in starting in the 2005 year
> of cars, "rear
[quoted text clipped - 26 lines]
> technicians using online discussion groups to troubleshoot,
> as well.)

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