Dual O2 Sensors and how they work?

[EagleMark]

Over the years of reading about how the rear O2 sensors work I've become nore confused or thought what I read was wrong. Now I think there might be 2 differant stratigies and hope someone can clear it up?

O2 sensor 1, bank 1 and 2. Both work the same way as OBDI with switch points and adjust fuel for closed loop.

O2 sensor 2, bank 1 and 2
1. Newer OBDII vehicles seem to use the second O2 sensor to check CAT efficiency with no CL fueling feedback?
2. Older OBDII vehicles second O2 sensor behind the CAT have some limited fueling correction? I can confirm this reading a 98 Vortec dissasembly as there is highs and lows for CL feedback on rear O2 sensors, also a bit for duel exhaust or single exhaust. Dual exhaust has dual CAT, Single exhaust has single CAT.

Reason I even got involved was my 98 Vortec has some fueling issues at idle. O2 sensor readings are:
Bank 1 sensor 1 switching and looks normal
Bank 2 sensor 1 switching and looks normal
Bank 1 sensor 2 steady voltage from 500 to 700 mv
Bank 2 sensor 2 voltage at idle swings like a sensor 1. When engine is reved it will go steady voltage. What alerted me to the issue was STFT and LTFT on bank 2 way off at idle only! It may be a old lazy O2 sensor or I have one bad CAT? Would a CAT go bad just at idle? I've done the on board O2 sensor tests and all pass!

At this point I think I could get better fueling from single O2 sensors? But will have to test disconnecting them and turning off diagnostic tests will allow front Pre CAT O2 sensors to work properly. Turning off diagnostic tests (not just MIL light) only and I still get readings from the post CAT o2 sensors.

[1project2many]

One bad cat for sure. Partly plugged or partly blown open can affect sensor temp and response. Did you have a misfire on bank 2 that might have caused partial meltdown?

Later vehicles will alter fueling from post cat O2. It's an oxygen in the catalyst thing.
Early do not or do less.
Unsure where early ends and later begins, though.

[EagleMark]

No misfires since I have owned it.

Injector spider was replaced along with intake gaskets, cap rotor plugs and a ton of other work about 6 months before I bought it. Total of recent receipts was $4000. and it also came with service records since new!

I just remembered when I got it I replaced 3 O2 sensors because I could not find records of them being changed and 3 is what the book showed.... taking a deeper look today there is one more post CAT O2 I did not change. This may be the issue?

Anyway to check CAT on vehicle?

[ony]

switch the o2s in the after cat position if the wires are long enough see if that bank has the same reading, if it has then the cat might have trouble. if it is a seperate duel ex. sys it may have one cylender running rich or lean.

[JeepsAndGuns]

Injector spider was replaced along with intake gaskets, cap rotor plugs and a ton of other work about 6 months before I bought it. Total of recent receipts was $4000. and it also came with service records since new!

Makes me wonder what was wrong with it to require all this work being done? Possiably the injector unit was bad or sticking on the side in question and therefore made the cat start going bad?

[EagleMark]

Makes me wonder what was wrong with it to require all this work being done? Possiably the injector unit was bad or sticking on the side in question and therefore made the cat start going bad?Not really hard to get to that amount with GM dealer service, all service on truck was what GM recommened at milage for life of truck without regard to if it needed it. Main issue was Intake gaskets were changed, fairly normal with DeathCool anti freeze, full flush and green stuff. Spider injector while it was out, oil cooler lines, transmission cooler lines, AC high pressure line, spark plugs, cap rotor... new tires and some other stuff. This did not include the new transmission 1000 miles before that. Truck now has 116k.

As a follow up I installed the other post CAT O2 sensor I missed the first time and the issue is gone.

[1project2many]

I just remembered when I got it I replaced 3 O2 sensors because I could not find records of them being changed and 3 is what the book showed.... taking a deeper look today there is one more post CAT O2 I did not change. This may be the issue?

Failure to provide all information can lead to incorrect diagnosis. ;) Although I don't usually run into an issue with a lazy heated, post cat O2 without a matching code on the vans. Are the O2 heater codes enabled in that cal?

[EagleMark]

Yes all codes enabled. ALso passed the O2 test.

Fueling STFT and LTFT are now balenced/same bank 1 and bank 2.

What the dissasembly says is post CAT O2 has high low adjusting of fueling of -74 mv to +74 mv.

[JP86SS]

To clarify this for me...
Sensor 1 is the Pre-Cat sensor
Sensor 2 is post-Cat sensor
Bank 1 = cylinders 1,3,5, & 7
Bank 2 = cylinders 2,4,6 & 8
That's the only way it seems to make sense to me but I'd like confirmation of that.
Yes?

[EagleMark]

Cylinders 1, 3, 5, 7
Bank 1 Sensor 1 - Driver's side, before catalytic converter
Bank 1 Sensor 2 - Driver's side, after catalytic converter

Cylinder 2, 4, 6, 8
Bank 2 Sensor 1 - Passenger side, before catalytic converter
Bank 2 Sensor 2 - Passenger side, after catalytic converter

I beleive all vehicles banks are the same. But cylinders are not. Lots of internet reference to bank 1 always being on side with cylinder number 1 is incorrect. Like Ford cylinder 1 is opisite chevy number 1 cylinder. But banks are same.

[POZE]

The bank and sensor numbers can be misleading. It would make sense that SAE would have standardized the locations knowing converters have Generic codes attached. I have guys tell me they disconnect O2s to and check PID data on scan tool to verify the faulty sensor location before replacement.

[Hog]

Look at this pic, left set of pipes is a Vortec 350 1/2 ton truck, the right pipes are from a 3/4 ton Vortec 350 truck.
http://i1202.photobucket.com/albums/bb373/Paul_Schermerhorn/l31exhaustback.jpg

On teh 1/2 tons the drivers side cat is actually fed by the passenger side bank of the engine,and pass. side cat is fed by drivers side engine bank.

On the 3/4 ton setup, the dirvers side cat is actually fed by the drivers side engine bank, and the passenger side cat is supplied by the pass side engine bank.

So which is bank 1 or Bank 2?

BTW Thos 1/2 ton pipes are horrible, those 90º bends on teh pipe from teh drivers side engine bank crimps down horribly, the uncrimped pipe measures 1 7/8". The 3/4 ton exhaust is 2 3/4" and des crimp down a bit, but no wheres near the 1/2 ton pipe.

This is why Longtube headers on teh 1/2 ton Vortec 350 trucks/SUV's add so much performance as you are forced to upgrade from teh tiny crimped headpipes. On a 3/4 ton truck you can get by with a set of good shorties.
The exhaust manifolds are EXACTLY the same between the 1/2, 3/4, 1 ton trucks/SUV's.

http://i1202.photobucket.com/albums/bb373/Paul_Schermerhorn/l31exhaustside.jpg
http://i1202.photobucket.com/albums/bb373/Paul_Schermerhorn/L31exhaustfront.jpg

peace
Hog

[JeepsAndGuns]

Wow, thats a pretty significant difference between trucks with the same engine.
If they have the same engine, and basicly the same truck, why not make them both have the same exhaust to cut down on production costs? Thos 1/2 ton pipes are tiny!

[EagleMark]

WOuld have been cheaper to just use 3/4 ton pipes... nice tech Hog!

[Fast355]

Cylinders 1, 3, 5, 7
Bank 1 Sensor 1 - Driver's side, before catalytic converter
Bank 1 Sensor 2 - Driver's side, after catalytic converter

Cylinder 2, 4, 6, 8
Bank 2 Sensor 1 - Passenger side, before catalytic converter
Bank 2 Sensor 2 - Passenger side, after catalytic converter

I beleive all vehicles banks are the same. But cylinders are not. Lots of internet reference to bank 1 always being on side with cylinder number 1 is incorrect. Like Ford cylinder 1 is opisite chevy number 1 cylinder. But banks are same.

That will get you in trouble on Nissan......Bank 1 on a V6 Nissan is the firewall or passenger side, RH side of the engine.

[Fast355]

Wow, thats a pretty significant difference between trucks with the same engine.
If they have the same engine, and basicly the same truck, why not make them both have the same exhaust to cut down on production costs? Thos 1/2 ton pipes are tiny!

My guess is the needed to get the cats to light off quicker in the 1/2 tons for emissions reasons. Some of the early 1/2 ton Express models had the 3/4 ton exhaust size setup.

[EagleMark]

That will get you in trouble on Nissan......Bank 1 on a V6 Nissan is the firewall or passenger side, RH side of the engine.
Well I guess we need a new chart for engines mounted in car sideways? Wonder if that is still true for American sideways engine?


My guess is the needed to get the cats to light off quicker in the 1/2 tons for emissions reasons. Some of the early 1/2 ton Express models had the 3/4 ton exhaust size setup.I wonder if it had to do with gearing too? Smaller exhaust does help with low end torque... for example if you want to screw up a perfect running Scout II take off the single exhaust and put dual exhaust on... they had heads that flowed worse then TBI heads so there was no need anyway.

[Fast355]

Well I guess we need a new chart for engines mounted in car sideways? Wonder if that is still true for American sideways engine?

I wonder if it had to do with gearing too? Smaller exhaust does help with low end torque... for example if you want to screw up a perfect running Scout II take off the single exhaust and put dual exhaust on... they had heads that flowed worse then TBI heads so there was no need anyway.

That line of engines started as school bus/medium duty truck engines. No need for flow when you have a 3,500-4,000 rpm governor and a 2bbl.

Nissans RWD V6 have bank 1 on the RH side too.

[Playtoy_18]

Bank 1 is always the bank that cylinder 1 is on.
front is sensor 1,after cat is sensor 2.
sensor 3 is usually LEV,ULEV,PZEV etc on the stuff I was trained on and it is generally the post cat sensor and sensor 2 will be installed in or directly before the cat.

not sure on the sensor 3 in gm offhand though.

years ago I read that GM does use the rear 02's to account for approx 1% of long term trims over time and deleting can cause strang transients.
i've deleted at least 50 over the past 6-7yrs and never had an issue though.

as far as 1/2 vs 3/4 ton cat position it prob has to do with different emissions standards and if it has an AIR system?

[daleulan]

Bank 1 is always the bank that cylinder 1 is on.
front is sensor 1,after cat is sensor 2.
sensor 3 is usually LEV,ULEV,PZEV etc on the stuff I was trained on and it is generally the post cat sensor and sensor 2 will be installed in or directly before the cat.

not sure on the sensor 3 in gm offhand though.

years ago I read that GM does use the rear 02's to account for approx 1% of long term trims over time and deleting can cause strang transients.
i've deleted at least 50 over the past 6-7yrs and never had an issue though.

as far as 1/2 vs 3/4 ton cat position it prob has to do with different emissions standards and if it has an AIR system?

The rear sensors are indeed used for fuel trims. There are three functions of those sensors in 'typical' software. I don't have a disassembly of GM's code in front of me but the most obvious is for catalyst diagnosis. The second function is for a continuous fuel trim adjustment to basically adjust for catalyst efficiency and desired feedgas composition. This won't affect fuelling much but to put it in perspective the front switching lambda sensors have an accuracy of around +/- 0.02 lambda; UEGO sensors are generally a bit worse. But put a switching lambda sensor behind the catalyst and the accuracy is about +/- 0.0005 lambda, with a significant improvement in emissions while allowing the engine calibrator to tune the engine for reasonable efficiency and power without sacrificing as much to get low NOx. The third function is one of active NOx recovery after fuel cut or deceleration recovery. This software basically starts when a lean transient is noted. Then the air-fuel ratio is richened up and the spark timing may be retarded (my software does it and I've needed spark retard here of up to 12 degrees to meet emissions) until the postcatalyst sensor snaps up to its normal voltage. In some versions of this algorithm the ECM determines the front-to-rear delay as a learned value so it can do the jumpback proactively. In any case the enrichment and the spark retard can affect drivability and certainly can affect emissions.

[skandolis]

i could be wrong, but when i was going to school(in the early nineties) we were taught that bank 1 was always the leading(frontmost) cylinder head (cylinders heads are always offset front to back) which makes cylinder 1 match up with bank 1 ect.........this has always proved to be the case to me.

[EagleMark]

Here's one that I'm not sure about and may change with systems?

Is the post cat O2 sensor supposed to switch? Or have steady reading?

[RobertISaar]

http://i.imgur.com/IGNCByl.jpg

pre vs post cat O2 sensor signals. the post-cat is simulated, but that's about what you could expect to see with a well functioning system.

[daleulan]

What you see out of the postcatalyst sensor can depend on the design OSC (cerium dioxide) loading of the catalyst. Early OBD-II catalysts used lighter OSC loading and so you would see an attenutated oscillation out of the rear sensor like above. Newer low-emissions catalysts have very high OSC loading and another interesting algorithm that I forgot about. The generaly target of most late model controllers is to maintain the postcatalyst sensor at a very steady target voltage - for gasoline it is commonly about 0.55 to 0.6 volts but may vary based on load and speed. Definitely the target changes with the fuel in use and also the NOx production of the engine. So on newer controllers you should see the postcatalyst sensor stay very steady. After a deceleration the voltage should return to steady-state quite quickly - within 5 seconds or so, maybe even quicker. You should see an enrichment in the front catalyst. The other sneaky algorithm is one that monitors the catalyst oxygen storage capacity and adjusts the front O2 sensor step-and-ramp to ensure that the postcatalyst sensor doesn't wiggle much.

[Playtoy_18]

Does anyody have any insights into the single bank front 02 on fwd 3800's versus the dual banks on the rwd's?

[Hog]

Playtoy is the FWD kinda like the Vortec 4.3/305?
I notice on the Vortec 4.3 L35 and Vortec 305 L30 that they use a 3 Precat O2 sensors, 1 for Bank 1, 1 for Bank 2 and then a 3rd O2 sesnor which reads BOTH Bank 1 and 2 mixed together just forward of the cat, then the aftercat O2 sensor.
The Vortec 350 L31 and Vortec 454L29/L21 use the conventional B1S1, B2S1, B1S2,B2S2 config with 2 precat and 2 postcat.

I remember something from Monodax about the post cat sensors affecting fueling. The acronym was FOASCO or something like that. I really wish we could access that info on Monodax, it was a great resource.

peace
Hog

[EagleMark]

I always wondered what that third O2 sensor was for in Vortec...

From what we have gathered and I partially found in a 98 Vortec Dissasembly the post cat O2 sensors do effect fueling up to 10%. That said I just did a 97 Vortec and both post cat O2 sensors were swinging! Both cats were shot. Disconnect and remove rear O2 sensors from cal and man did it run better.

[Playtoy_18]

No, except for the camaro most 3800's I've seen have a single 02 sensor 1, and single 02 sensor 2.
So it has only a single 02 sensor for fueling both banks and one after cat for monitors.

Most vortecs I've dealt with have 4 02 sensors.
Precat/fueling is bank 1 sensor 1 and bank 2 sensor 1
Postcat/monitor is bank 1 sensor 2 and bank 2 sensor 2

Low emission ulev, pzev etc would have like 3 sensors total for say a 4cyl.
Precat/fueling would be bank 1 sensor 1 then there will be another installed in cat towards the front and another that is post cat.
These would be bank 1 sensors 2 and 3, but not necessarily in order "I THINK".
I suspect sensor 2 might be always post cat monitor and sensor 3 is in the middle designated for whatever makes it a pzev.
Confused me everytime, had to look in book and verify etc every time they rolled into shop.
There is an order and it is not necessarily IN order I think.
I took a class long ago but forget it honestly, not a lot came thru our place.