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Thread: Commanding 15.34 AFR?

  1. #1

    Commanding 15.34 AFR?

    Why my tune commanding a 15.34 AFR?

    It happens around 1:25 into the log. I also see it oddly command 13.0 right after this event before commanding the expected 14.08.

    I don't see why any adders would be triggered here and light throttle/rpm/map changes. Happens in other areas as well.
    - ect 176
    - iat 75


    SD V6.1.hpl

    C10 Gen3 6.2 V6 Cleanup.hpt

  2. #2
    Tuning Addict 5FDP's Avatar
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    It's a cat test. Happens when you remove the cats and the rear o2 sensors are completely gone.

    When I had a 2006 truck that I did the offroad Y-pipe on, I just plugged the rear sensors back in and zip tied them to the frame, that stopped the test from going.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  3. #3
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    Definitely don't take the rear o2's out of the exhaust.. There are some odd issues that pop up doing that. Maybe not on that 06 truck but for sure later ones will have a fit over that..
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  4. #4
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    I have an L31 350 and a 8.1L that run that same catalyst test and they have cats and rear 02s. They all do it from my experience. It is not a weird issue, it is literally what the PCM is programmed to do to test catalyst efficiency.

  5. #5
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    Quote Originally Posted by 5FDP View Post
    It's a cat test. Happens when you remove the cats and the rear o2 sensors are completely gone.
    ive gotten into countless arguments about this.
    i tell poeple all the time, leave the rear o2s. zip tie them to the frame or add bungs and screw them into the exhaust, but leave them hooked up.

    so many just want to argue and not listen to facts

  6. #6
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    Quote Originally Posted by bk2life View Post
    ive gotten into countless arguments about this.
    i tell poeple all the time, leave the rear o2s. zip tie them to the frame or add bungs and screw them into the exhaust, but leave them hooked up.

    so many just want to argue and not listen to facts
    There are GM vehicles that did not have rear 02s from the factory usually higher GVW 3500 stuff or Kodiak 4500+. My 97 Express with a L31 never had rear 02s from GM. There is a factory cap on the engine harness where the rear 02 sub harness would have plugged into it. There are also some vehicles that did not have front 02s and ran open loop only, usually export leaded fuel compatible but the US forest service had some too. I have run across a LQ4 Suburban and L31 Express with P01s that had factory open loop tunes that were US Forest Service vehicles built without catalysts and 02s. The catalyst efficiency test runs with the rear 02s and cats in place too. The catalyst test can be eliminated all together with open source for non-emissions controlled swap vehicles. There is no reason to keep the rear 02s without catalysts in a swap environment. The 02/catalyst test last all of 10 seconds anyway with the rich then lean sweep. The PCM varying the AFR 1 point either direction from stoich at idle for a few seconds is not hurting anything anyway.
    Last edited by Fast4.7; 05-31-2025 at 01:48 PM.

  7. #7
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    Quote Originally Posted by Fast4.7 View Post
    The catalyst test can be eliminated all together with open source for non-emissions controlled swap vehicles.
    id be intersted in knowing more about this.
    if its open source, please point me in the direction, or for more discreet, a message. I do non emission swaps on exempt vehicles

  8. #8
    Okay, I have a standalone harness which has the rear O2's eliminated so I'm gonna just go ahead and ignore it.

    Happens at light cruise speeds and lasts only a few seconds so I'll deal with it until a solution presents itself.

  9. #9
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    Most of the time it's unnoticeable
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