O2 Sensor failover???

Discussion in 'Engine Performance + Maintenance' started by Motorsports-X, Jul 31, 2020.

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  1. Motorsports-X

    Motorsports-X Newbie

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    Ok so...first off...id never hack up a truck like this.

    I was able to find 3 o2 sensor plugs. On the left side of the truck i found a flat 4 wire plug. Bought a flat four wire direct fit o2 senso . When i plug it in i do see B1S1 come alive on my data stream. Unplug it and it's steady 450 mv. The other 2 wires are both square plug. So i went and also bought a square plug direct fit upstream sensor. I figured one of them must be upstream. Nope. I plug then in and get b1s2 and b2s2 readings in the data. I cant even find a 4th plug anywhere in the truck. Nowhere the harness is cut.. not stuffed on top of the tranny... there is just no 4th plug i can find. Anyone know if there is supposed to be one and where it may come out of the harness?

    So i took the passenger inner fender out to fix that cluster f of wires in there and pull that heat shield out (wasnt mounted and rattling around) no plug there either btw.. but i cant drive it just yet to cycle the emmisions. However i got 0 volts on the 02 sensor at idle. Is it possible im to far away from the block? The pipe comes out of the shorty header and crosses over to the passenger side then turns 90 and heads back. The sensor is about 6 inches or so after the bend. It does run up to .9 volts on start but then falls right down to 0-.1. Giving it throttle makes no difference. Also there is no trim after idling and simulating load and putting it in gear and giving it gas while i hold the brake. At this point im wondering if its mandatory both s sensors are connected and where that 4th plug is. Screenshot_20200801-184805_RepairSol2.jpg Screenshot_20200801-183116_RepairSol2.jpg Screenshot_20200801-182911_RepairSol2.jpg
     
  2. Schurkey

    Schurkey I'm Awesome

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    WHAT ENGINE? My '97 7.4L has four O2 sensors, but I'm thinking the 5.7 has three. Maybe I'm wrong, maybe it depends on the vehicle.

    Check your service manual for harness routing.
     
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  3. RawbDidIt

    RawbDidIt I'm Awesome

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    For reference, my 97 k1500 5.7 has 4 as well. Upstream has the square connectors you mentioned, downstream has the 4 in-a-line configuration for the plug.

    Sent from my SM-N976V using Tapatalk
     
  4. Motorsports-X

    Motorsports-X Newbie

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    Mine has the 4 in a row upstream. The square ones are down stream. I have all of them working except b2s1. And the only code i have now is for that sensor. Apparently the ecm will not go into closed loop without both sensors. I drove 40 miles today and got no fuel trim at all. Im going to try to find that plug again tommorrow or I'll rewire it from the pcm. Ready to get this 411 in.
     
  5. Motorsports-X

    Motorsports-X Newbie

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    Yesterday when i put in the vin.. i oddly saw a sensor "3" line in the data stream selector. Thought it was a glitch. But i just noticed thst the 5.0 had a b1s3. This truck had a 5.0 in it originally. I think that's why i cant find the plug. When the ecm was swapped to the 350 ecm it just happened that the existing o2 sensor wiring lined up with b1s1 b1s2 and b2s2. This is just a theory. I need to verify how many sensors a 5.0 had and the pinout differences. I should just be able to swap the pins.
     
  6. arrg

    arrg I'm Awesome

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    5.0 should have 4 sensors. Bank 1 S1, Driver's side downpipe. Bank 1 S2, after the y-pipe, pre-catalytic converter, Bank 1 S3 post-catalytic converter, Bank 2 S1 passenger's side downpipe.
     
  7. Postmech

    Postmech Newbie

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    My 1998 5.0 Vortec came with 1 cat and one downstream oxygen sensor. Most other 305's I have seen with 49 state systems have a y pipe and one cat like mine.
     
  8. Motorsports-X

    Motorsports-X Newbie

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    downloaded the service manuals.. found the plug. it was shoved up towards the top of the left side of the engine, under another harness. its a 4 blade flat. so thats done.... lol the plug for b2s2 is closer to where the sensor actually is, and the second sensor i bought is sqaure plugged, So i am going to re-pin the harness temporarily. Hopefully we get some trim numbers tonight!!

    fyi, both the 5.0 and 5.7 have down 4 o2 sensors. however, the 5.0 only has one down stream sensor (b1s3) and 1 cat. the 5.7 has induvidual cats and 2 down stream sensors.
     
  9. RawbDidIt

    RawbDidIt I'm Awesome

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    So the 305 has 3 upstream o2 sensors?

    Sent from my SM-N976V using Tapatalk
     
  10. arrg

    arrg I'm Awesome

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    Yes. B1S1 and B2S1 control fueling. B1S2 and B1S3 are to monitor catalyst efficiency.
     

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