Catalytic Converters

[clare]

went through a incredible class a few years ago concerning rear 02 sensors
and fuel trim. Even tho the manufactured say it won't, or in some cases will
say "minorly" it can major effect fuel trim/gas mileage.
its the difference in what the tell you, and "us" the manufacture tech's and
what really happens.
Steve

Explain how the catalyst monitor affects fuel mileage. I'm all ears.

 

[S.Hansen]

Explain how the catalyst monitor affects fuel mileage. I'm all ears.

LOL, I'll see if I can find the original post.

Here it is, I cut some of the non applicable stuff.

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.

 

[Todd H.]

Explain how the catalyst monitor affects fuel mileage. I'm all ears.

Maybe the monitoring actually is used as an input to the ECU that does
something with fuel trim on his particular model and year?

Theory or not, we have one person on the thread write
"I had the 420 code, put in a new rear O2 sensor, and the CEL
went away. My mileage also increased from about 26 mpg to about
28 mpg."

and another indicating having learned that there's anecodotal evidence
that suggests this can actually happen.

On the other hand, you're swearing up and down that changing a rear O2
can't possibly be causally linked to a change in fuel economy. I'm
not sure the burden of proof is on the others. Now, maybe you are
omniscient on the topic and know for a fact that every design of his
model, year and engine has a rear O2 that does NOTHING to adjust any
parameters that'd affect fuel economy, and the other two data points
are invalid.

But my reading on the subject suggests that the rear O2 does affect
air fuel mixture, just not nearly as directly as the front does.

 

[Todd H.]

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


--Catalyst efficiency monitor?


That's only a portion of its function


REAR FUEL TRIM
....
Every vehicle manufacture today uses the rear O2S for fuel
correction; even if it is undocumented.

Ohhhh snap.

 

[clare]

Maybe the monitoring actually is used as an input to the ECU that does
something with fuel trim on his particular model and year?

Theory or not, we have one person on the thread write
"I had the 420 code, put in a new rear O2 sensor, and the CEL
went away. My mileage also increased from about 26 mpg to about
28 mpg."

and another indicating having learned that there's anecodotal evidence
that suggests this can actually happen.

On the other hand, you're swearing up and down that changing a rear O2
can't possibly be causally linked to a change in fuel economy. I'm
not sure the burden of proof is on the others. Now, maybe you are
omniscient on the topic and know for a fact that every design of his
model, year and engine has a rear O2 that does NOTHING to adjust any
parameters that'd affect fuel economy, and the other two data points
are invalid.

But my reading on the subject suggests that the rear O2 does affect
air fuel mixture, just not nearly as directly as the front does.

From what I had read previously, those pre-obd2 vehicles that used
secondary O2 sensors could use them for fuel trim to optimize the cat,
but it was my understanding the OBD2 standard did not "allow" this.
However, it is becoming obvious nobody adheres to "standards"

You'd think we were talking about the computer industry - - -

 

[clare]

LOL, I'll see if I can find the original post.

Here it is, I cut some of the non applicable stuff.

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.

I'd question the "every vehicle"

Also, the "abnormal conditions" may have included some other
malfunction???

2% is 1/4 mpg at 25 mpg mean. and I'd grant you that might be normal
(and also virtually undetectable to the average driver/owner, and even
mechanic.)

 

[S.Hansen]

I'd question the "every vehicle"

Also, the "abnormal conditions" may have included some other
malfunction???

2% is 1/4 mpg at 25 mpg mean. and I'd grant you that might be normal
(and also virtually undetectable to the average driver/owner, and even
mechanic.)

This is what the class was about. using fuel trim to analyze the car. This
class was in late 2005. At this time Ford's scantool was the only one that
was offering a pid for "rear fuel trim" and using that info the class did a
case study on a car that had excessive fuel consumption. the Ford shop
looked at it twice, and using standard diag techniques it read OK (Short
term+long term= indication of fuel consumption) said it was OK. But, by
adding in the rear fuel trim numbers suddenly the car was not OK. Ended up
having excessive alcohol in the fuel.
I honestly think all card use this, but don't document it even now.

 

[Remco]

That's what I figured as well. The O2 sensors were the easiest to
replace so did them first.

Well, here it is two weeks later and the P0420 code is back. At least
it passed emissions.
Both O2 sensors and the CAT have been replaced.

 

[clare]

This is what the class was about. using fuel trim to analyze the car. This
class was in late 2005. At this time Ford's scantool was the only one that
was offering a pid for "rear fuel trim" and using that info the class did a
case study on a car that had excessive fuel consumption. the Ford shop
looked at it twice, and using standard diag techniques it read OK (Short
term+long term= indication of fuel consumption) said it was OK. But, by
adding in the rear fuel trim numbers suddenly the car was not OK. Ended up
having excessive alcohol in the fuel.
I honestly think all card use this, but don't document it even now.

It was the excess alcohol that made the rear sensor come into play,
i'd venture to bet. Running E85 in a non-flex-fuel engine can cause
all kinds of anomolies.
Under normal operating conditions a rear O2 sensor has very minimal
mixture authority - and I believe that is dictated by the OBD2
standard.

 

[clare]

Well, here it is two weeks later and the P0420 code is back. At least
it passed emissions.
Both O2 sensors and the CAT have been replaced.

Factory or aftermarket parts?????
If aftermarket all bets are off - period.
The replacement parts market is so full of CRAP today that you can
easily cause more problems than you fix by installing new parts.

 

[Carl 1 Lucky Texan]

Well, here it is two weeks later and the P0420 code is back. At least
it passed emissions.
Both O2 sensors and the CAT have been replaced.

I know there are cheap aftermarket cats. And some folks have reported
problems with using Bosch sensors. But I can't help wondering if there
could be another issue. I suppose the wiring is good? Are there any
other codes stored?

carl

 

[S.Hansen]

From what I had read previously, those pre-obd2 vehicles that used
secondary O2 sensors could use them for fuel trim to optimize the cat,
but it was my understanding the OBD2 standard did not "allow" this.
However, it is becoming obvious nobody adheres to "standards"

You'd think we were talking about the computer industry - - -

Think the other way when it comes to OBD standards. The Manufactures can do
whatever they want in there cars as long as certain conditions are met.
Emissions to a certain level. Usually modeling after California's standards
that are higher than the rest of the world.
Certain Info available to the general shop; The EPA and all the biggie-wigs
in Washington decide what Pids are available to the aftermarket world, OBD
Generic. IE, several years ago Misfire Data wasn't available except on OEM
scanners, now it is more common on the generic lines. IF a manufacture
decided to give more info to the aftermarket world, they could. A prime
example was Chrysler. They had ton's more info available on a aftermarket
scanners years before other car manufactures.
Even though OBD is a rule, interpretation of the rules vary by manufacture.
The 1996 rules for the obd data plug said something along the line of "Must
be located 12" from the centerline of the dash, visible by the technician
from the drivers door area". How can hidden under a ashtray be classified as
"Visible" (Honda) Some were on the passenger side of the dash.

The rules are up for grabs.

 

[Remco]

 Factory or aftermarket parts?????
If aftermarket all bets are off - period.
The replacement parts market is so full of CRAP today that you can
easily cause more problems than you fix by installing new parts.

Yeah, I heard about those issues so replaced them with genuine Subaru
parts..
Maybe I should have gone with aftermarket..

 

[Remco]

I know there are cheap aftermarket cats. And some folks have reported
problems with using Bosch sensors. But I can't help wondering if there
could be another issue. I suppose the wiring is good? Are there any
other codes stored?

carl

I checked the wiring. It looks good. I also checked the snug-ness of
the sensors, thinking I didn't tighten it enough. It is good.
For that thing to throw a efficiency code, one would imagine that it
measures both O2 sensors and, by way of the ratio of one measurement
over the other, it decides that the ratio is good/not good.
Googling around, it appears some people have put resistors in place of
the sensors to get rid of the condition. I wonder if you can pass
emissions with them things in place. Don't they measure the O2
sensor's output when they test your emissions?

 

[Carl 1 Lucky Texan]

I checked the wiring. It looks good. I also checked the snug-ness of
the sensors, thinking I didn't tighten it enough. It is good.
For that thing to throw a efficiency code, one would imagine that it
measures both O2 sensors and, by way of the ratio of one measurement
over the other, it decides that the ratio is good/not good.
Googling around, it appears some people have put resistors in place of
the sensors to get rid of the condition. I wonder if you can pass
emissions with them things in place. Don't they measure the O2
sensor's output when they test your emissions?

If you dig around at www.nasioc.com , you can find out about the
resistors and, IIRC, a 'spacer' that also helps with the code. I would
prefer to fix it 'correctly' myself - unless i was building a track car
or something....

Sigh....it's a bummer not knowing why you still get this code. maybe the
ECU is bad?


good luck

Carl

 

[Remco]

If you dig around atwww.nasioc.com, you can find out about the
resistors and, IIRC, a 'spacer' that also helps with the code. I would
prefer to fix it 'correctly' myself - unless i was building a track car
or something....

Sigh....it's a bummer not knowing why you still get this code. maybe the
ECU is bad?

good luck

Carl

I'm with you: I prefer to fix things right. Two years from now, should
she need to pass emissions again, I'll have to temporarily hack it to
get by.

 

[clare]

I checked the wiring. It looks good. I also checked the snug-ness of
the sensors, thinking I didn't tighten it enough. It is good.
For that thing to throw a efficiency code, one would imagine that it
measures both O2 sensors and, by way of the ratio of one measurement
over the other, it decides that the ratio is good/not good.
Googling around, it appears some people have put resistors in place of
the sensors to get rid of the condition. I wonder if you can pass
emissions with them things in place. Don't they measure the O2
sensor's output when they test your emissions?

A resistor will not do it. The front sensor acts like a battery, with
higher oxygen content giving a higher voltage. It has to swing high
and low from the .45 volt center. If it doesn't, the system cannot
correct mixture and it throws a code. To make the cat work properly it
NEEDS to transition from rich to lean. If it never goes rich the one
part of the cat will not work. If it never goes lean the other part
will not work.

The rear sensor monitors the converter. Ideally it will NOT follow the
front sensor at all because the converter in an ideal world would even
out the O2 level of the exhaust. If the converter is not working
properly the rear sensor transitions more - and if it is not working
at all it will follow the front sensor exactly.

Up hear in Ontario if the CEL is on it doesn't pass - and with a
resistor the CEL would come on. Other than that the PCM is not queried
- but if the cat is not working up to snuff it won't pass anyway, so
it is a moot point.

 

[Remco]

A resistor will not do it. The front sensor acts like a battery, with
higher oxygen content giving a higher voltage. It has to swing high
and low from the .45 volt center. If it doesn't, the system cannot
correct mixture and it throws a code. To make the cat work properly it
NEEDS to transition from rich to lean. If it never goes rich the one
part of the cat will not work. If it never goes lean the other part
will not work.

The rear sensor monitors the converter. Ideally it will NOT follow the
front sensor at all because the converter in ...

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I only mentioned that people have gotten by hacking it up with some
sort of resistor circuitry.
Note I'm not suggesting it as a fix.

It is disconcerting that it spews a vague error like that. It implies
that it has to do with the efficiency of the cat so either it is due
to the way the cat's efficiency is monitored or the cat itself.
In my case, I've replaced all those components, yet the error comes
back like a boomerang. Clearly something else can cause this error to
be thrown.

Of course, the front O2 sensor was indeed bad. It didn't change when
the car was driven.

 

[clare]

I only mentioned that people have gotten by hacking it up with some
sort of resistor circuitry.
Note I'm not suggesting it as a fix.

It is disconcerting that it spews a vague error like that. It implies
that it has to do with the efficiency of the cat so either it is due
to the way the cat's efficiency is monitored or the cat itself.
In my case, I've replaced all those components, yet the error comes
back like a boomerang. Clearly something else can cause this error to
be thrown.

Of course, the front O2 sensor was indeed bad. It didn't change when
the car was driven.

The sensor being bad isn't the only thing that will make it not
"switch". If the engine is too rich, or too lean, or has an exhaust
leak ahead of the sensor, it will read as bad. Other possible causes
as well.

Also, the converter does not need to be BAD to not work. If it never
gets lean enough or rich enough to function properly it will fail it's
efficiency test.The nasty exhaust leak before the converter can and
will do this.

 

[Remco]

On May 28, 4:23 pm, (e-mail address removed) wrote:
..

The sensor being bad isn't the only thing that will make it not
"switch". If the engine is too rich, or too lean, or has an exhaust
leak ahead of the sensor, it will read as bad. Other possible causes
as well.

Also, the converter does not need to be BAD to not work. If it never
gets lean enough or rich enough to function properly it will fail it's
efficiency test.The nasty exhaust leak before the converter can and
will do this.

I'll crawl under the car again and check that out. I didn't see any
before but maybe I missed it.
Thanks.