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Upstream vs downstream

Discussion in 'Technical Chat' started by 80schild, Sep 17, 2017.

  1. Sep 17, 2017 at 8:45 AM
    #1
    80schild

    80schild [OP] Well-Known Member

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    My truck has long tube headers and just yesterday I started getting a P2241 code which is for bank 2 sensor 1. After getting under the truck and looking I noticed both sensors on each side are forward of the cat... is it supposed to be that way? I thought sensor 2 was supposed to be behind the cat?

    I had a dead battery a few days ago and changed it out, and honestly I never had a CEL until after that. No idea if it's related. :confused:

    (Pic is of driver side, but passenger side is the same way)

    image.jpg
     
  2. Sep 17, 2017 at 9:08 AM
    #2
    Redleg06

    Redleg06 Member

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    My original exhaust manifolds had the cat built in, I imagine yours were the same.
     
  3. Sep 17, 2017 at 9:33 AM
    #3
    windsor

    windsor Just a guy

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  4. Sep 17, 2017 at 10:38 AM
    #4
    80schild

    80schild [OP] Well-Known Member

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    Windsor, yours looks set up identical to mine. I just can't help but wonder if it throws off the readings, making the ecu think the cats aren't working. You never get any codes with yours?
     
  5. Sep 17, 2017 at 11:07 AM
    #5
    Redleg06

    Redleg06 Member

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  6. Sep 17, 2017 at 11:26 AM
    #6
    80schild

    80schild [OP] Well-Known Member

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  7. Sep 17, 2017 at 1:42 PM
    #7
    Redleg06

    Redleg06 Member

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    No, that's to give the computer a signal that makes it think the second o2 sensor is downstream from the cats. I have the same setup you and windsor have and no codes.
     
  8. Sep 17, 2017 at 1:45 PM
    #8
    windsor

    windsor Just a guy

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    The original exhaust manifolds had cats in them with one sensor before the cat (upstream) and the other in the y-pipe (downstream). By changing to the long tube headers, you no longer have the cats between the sensors. The URD o2 sims correct the signal and make the computer think the OE parts are there and working.
    s-l225.jpg
     
  9. Sep 17, 2017 at 4:01 PM
    #9
    80schild

    80schild [OP] Well-Known Member

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    Ah, I see. So it basically just fools the ECM.
     
  10. Sep 17, 2017 at 4:08 PM
    #10
    windsor

    windsor Just a guy

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  11. Sep 20, 2017 at 8:30 AM
    #11
    80schild

    80schild [OP] Well-Known Member

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    Well, I crawled under my truck, and I should have already noticed (as seen in the pic) that my truck already has the URD o2 simulator. I bought one from a member here, so I guess I'll just put it in a drawer in case my old one ever fails.

    So, back to my bank 2 sensor 1 code... Everywhere I look at the Denso 234-9026 A/F sensor it always says "upstream left". Are the bank 1 and bank 2 upstream sensors the same? I don't see a special one listed for bank 2.
     

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