Q about placement of wideband A/F sensor

[LtNoogie]

I just wanted to get this straight in my head. The further back one places the wideband A/F sensor, in the exhaust system, the leaner it will show? i.e. most accurate is in the header, more lean after first cat, more lean after second cat.

The reason I ask is that when I dyno'd the car, the mechanic read the A/F mixture from the tailpipe. The entire A/F curve is probably shifted slightly to the lean side, right?

TIA

[eTiMaGo]

I would think the tailpipe sensor has some way to compensate for the catalytic effect? Not really knowledgeable enough to give you an exact answer on that, but it does get a reading based on the oxygen amount in the exhaust, and that *should* be unaffected by the cats...

But if you're installing your own sensor, definitely get it as close to the header collector as possible

[richardholdener]

The position of the oxygen sensor in the exhaust system will not change the reading (rich or lean). The farther away from the cylinder head, the air/fuel curve will have basically a time lag when compred to engine speed. This means that the curve will be the same with respect to rich or lean, it will be just shifted slightly in the rpm range. Once agin, no change in air/fuel reading from change in position of oxygen sensor or from having catalytic converter in system (seems odd but back to back tests reveal identical air/fuel before and after cat).

I just wanted to get this straight in my head. The further back one places the wideband A/F sensor, in the exhaust system, the leaner it will show? i.e. most accurate is in the header, more lean after first cat, more lean after second cat.

The reason I ask is that when I dyno'd the car, the mechanic read the A/F mixture from the tailpipe. The entire A/F curve is probably shifted slightly to the lean side, right?

TIA

[LtNoogie]

Thanks Thomas and Richard.

[largeorangefont]

The position of the oxygen sensor in the exhaust system will not change the reading (rich or lean). The farther away from the cylinder head, the air/fuel curve will have basically a time lag when compred to engine speed. This means that the curve will be the same with respect to rich or lean, it will be just shifted slightly in the rpm range. Once agin, no change in air/fuel reading from change in position of oxygen sensor or from having catalytic converter in system (seems odd but back to back tests reveal identical air/fuel before and after cat).

I agree. I have seen slight variances at idle when taking AFR readings pre and post cat, but these seem to go away once the car is moving.

I've actually tested with 2 widebands at the same time.

Are you going to run a wideband in the car or do you just want a bung for testing?

[LtNoogie]

I actually had a bung installed between the two cats for my wideband A/F. I was debating whether to spend the extra money to have the muffler shop drop the midpipe so that they could install another bung in the "correct" place in front of the first cat. If the meter is not going to show an appreciable difference, I'll start running the harness into the interior.

[largeorangefont]

I actually had a bung installed between the two cats for my wideband A/F. I was debating whether to spend the extra money to have the muffler drop the midpipe so that they could install another bung in the "correct" place in front of the first cat. If the meter is not going to show an appreciable difference, I'll start running the harness into the interior.

Do you still have the stock cats? If so I would consider getting another bung up stream of the cats. If they are high flow aftermarket cats, don't worry about it.

In either case you will probably be OK. It is all relative anyway. No matter where you have the sensor mounted, if your readings change one day out of the blue, you will know something is amiss.

[GeneW]

I would put it before the Cat myself. The cat changes the chemistry of the mixture big time.

Gene

[The Spectacle]

The position of the oxygen sensor in the exhaust system will not change the reading (rich or lean). The farther away from the cylinder head, the air/fuel curve will have basically a time lag when compred to engine speed. This means that the curve will be the same with respect to rich or lean, it will be just shifted slightly in the rpm range. Once agin, no change in air/fuel reading from change in position of oxygen sensor or from having catalytic converter in system (seems odd but back to back tests reveal identical air/fuel before and after cat).

I know this is old but I stumbled across this post in a google search and I feel obligated to post...this is plain false. The futher away from the motor (or conversely the closer to atmosphere you place) the o2 is the leaner it will show relative to another o2 placed further upstream in the exhaust system. Tailpipe sniffers on dyno's have free air going past them and will read leaner.

[127.0.0.1]

I would think the tailpipe sensor has some way to compensate for the catalytic effect? Not really knowledgeable enough to give you an exact answer on that, but it does get a reading based on the oxygen amount in the exhaust, and that *should* be unaffected by the cats...

But if you're installing your own sensor, definitely get it as close to the header collector as possible


the A/F sensor must be located in the raw exhaust stream before the first cat.

cat converters would render the readings meaningless

an O2 sensor is located after the cats to check for the efficiency of the cats themselves

[cali yaris]

^ no, it's located between the two cats. So it can't be checking the second cat.

[127.0.0.1]

^ no, it's located between the two cats. So it can't be checking the second cat.

EDIT: lawls...so wrong.

on these things it is between ? that's a new one on me


vehicles with 2 cats the a/f is always between engine and 1st cat

and the mid sensor is o2, which checks the cat 1 function, and the last
sensor is o2 also, also that checks the cat2 function. [almost no cars
have 3 sensors unless it multibank]

[127.0.0.1]

http://etimago.com/yaris/repairmanual/Emission%20Control.pdf


the air/fuel sensor is always *before* the cats as in all cars that use a/f sensors


the o2 sensor role (in cars the use a/f sensors) is to verify cat operations...and that is placed after a cat

in cars without a/f sensors, the front o2 sensor would be where the a/f sensor would be, and does
a similar job but with less accuracy. more like a rich/lean on/off switch than a linear response

[127.0.0.1]

The position of the oxygen sensor in the exhaust system will not change the reading (rich or lean). The farther away from the cylinder head, the air/fuel curve will have basically a time lag when compred to engine speed. This means that the curve will be the same with respect to rich or lean, it will be just shifted slightly in the rpm range. Once agin, no change in air/fuel reading from change in position of oxygen sensor or from having catalytic converter in system (seems odd but back to back tests reveal identical air/fuel before and after cat).


never put an engine controlling air/fuel sensor behind a cat. you do not want the computer running the engine based on reading from behind a cat.

[jaherrera1]

my english y bad.

My yaris (TOYOTA YARIS (NCP90L-AGMRK)) have code P0031 HO2S Heater Control Circuit Low (Bank 1 Sensor 1). it has two oxygen sensors, which one should I replace according to the diagram below ...

http://www.toyodiy.com/parts/p_G_200711_TOYOTA_YARIS_NCP90L-AGMRK_8401.html?hl=89465A

which one is Bank 1 sensor 1... the part number 89465-52370 or 89465-52380 number

[Bluevitz-rs]

Sensor 1 is the one closest to the engine, in the header.