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Thread: Dual O2 Sensors and how they work?

  1. #1
    RIP EagleMark's Avatar
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    Dual O2 Sensors and how they work?

    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.

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    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.

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    RIP EagleMark's Avatar
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    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?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
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  4. #4
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    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.

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    Fuel Injected! JeepsAndGuns's Avatar
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    Quote Originally Posted by EagleMark View Post
    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?
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

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    RIP EagleMark's Avatar
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    Quote Originally Posted by JeepsAndGuns View Post
    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.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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    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?

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    RIP EagleMark's Avatar
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    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.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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    Fuel Injected! JP86SS's Avatar
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    Lightbulb

    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?
    86 Monte, 406, Hyd Roller, 700R4 beefed, G3-APU1 and NVSRAM 730, S_AUJP

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    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.

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  11. #11
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    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.

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    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.


    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.




    peace
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  13. #13
    Fuel Injected! JeepsAndGuns's Avatar
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    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!
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  14. #14
    RIP EagleMark's Avatar
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    WOuld have been cheaper to just use 3/4 ton pipes... nice tech Hog!

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
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  15. #15
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    Quote Originally Posted by EagleMark View Post
    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.

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