P0171 - Recently replaced pistons & rings

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by djimbuser, Oct 27, 2025 at 2:31 PM.

  1. djimbuser

    djimbuser Member

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    Prior to tearing engine down and replacing pistons & rings there was no P0171 code. After getting the engine reassembled I get the P0171 on every second drive (service manual clearly states it's a two-drive code).

    What I can't figure out is why. All hoses connected. Upstream O2 sensor connected. Fittings between the MAF and throttle body have tight clamps.

    I captured the freeze frame data and attached them can anyone single out the issue on the freeze frame data?

    Thank you!
     

    Attached Files:

  2. PriusCamper

    PriusCamper Senior Member

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    Wiggle all the vacume hoses... Maybe one of them is damaged?
     
  3. djimbuser

    djimbuser Member

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    Wish it was that simple. All the hoses seem flexible and in good shape. I took off the air cleaner box and noticed that one of the clamps to the duct leading to the throttle body wasn't tight so I tightened that and thought maybe that was the fix but I drove it two more drives and the CEL reappeared :(
     
  4. Brian1954

    Brian1954 Senior Member

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    Check the hose from the PVC valve to the intake manifold. Also, check the two hoses that I wrote about in a different thread two weeks ago which I copied below.
     
    #4 Brian1954, Oct 27, 2025 at 5:58 PM
    Last edited: Oct 27, 2025 at 8:25 PM
  5. djimbuser

    djimbuser Member

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    I have ordered a new upstream O2 sensor I'm going to try that... That said all I did was disconnect those three connections down near bottom of timing of timing cover so I could sling the wiring harness up and out of the way during the engine work days. I'm 100% sure I've reconnected it correctly.
     
  6. djimbuser

    djimbuser Member

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    Oh yes I HATE that hose. Backstory on this car I replaced the head gasket a couple years ago and that hose split at the top so I cut it off and used a hose clamp. I did not have a P0171 CEL for doing that...

    Update/clarification. The hose IS connected to the top engine cover.. it's just that so little of the hose is on the nipple I thought it best to use a clamp to make sure it doesn't fall off.
     
  7. djimbuser

    djimbuser Member

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    I see it now.. it's the "Total FT #1" on image #2. My car doing 39% which is outside of the acceptable high of 35%... Now the question is WHY. I have new Denso Fuel injectors, new Denso coils, new NGK spark plugs, new EGR Valve, and super clean EGR heat exchanger.
     
  8. Brian1954

    Brian1954 Senior Member

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    You have a vacuum leak. Remove the air box and check the three hoses I mentioned in post #4.
     
    #8 Brian1954, Oct 27, 2025 at 8:13 PM
    Last edited: Oct 27, 2025 at 8:19 PM
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  9. Brian1954

    Brian1954 Senior Member

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    That is not one of the three hoses that I was talking about in post #4.
     
  10. BiomedO1

    BiomedO1 Senior Member

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    And it may not be a hose, but a ruptured diaphragm - same as a disconnected hose. You need to use a vacuum pump to check them.o_O:eek:
    That was week of my life that I'll never get back.:cry::whistle:
     
  11. Brian1954

    Brian1954 Senior Member

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    What diaphragm are you talking about on a Gen 3 Prius?
     
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  12. BiomedO1

    BiomedO1 Senior Member

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    Well, if a gen3 Prius doesn't have vacuum controlled diaphragms - forget it. I'm used to working on the really old stuff that use to have those. The rubber would get old and rupture internally; so everything looks good upon visual inspection.
     
    #12 BiomedO1, Oct 27, 2025 at 8:34 PM
    Last edited: Oct 27, 2025 at 8:40 PM
  13. djimbuser

    djimbuser Member

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    Ok.. I'll report back tomorrow!