Catalytic Converters

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.

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.

2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA

...

Ohhhh snap.

-- Todd H.

2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA

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 - - -

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.

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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.

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.

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.

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

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.

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

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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

, 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

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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.

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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.

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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.

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CEL.  Had it

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.

On May 28, 4:23=A0pm, snipped-for-privacy@snyder.on.ca wrote: .

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