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Another elusive P0171

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Marioaz, Jun 23, 2022.

  1. Marioaz

    Marioaz Junior Member

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    Hi All,

    I have noticed a higher RPM on idle while in Park and a reduced MPG up to 48 city driving and lower highway. After a while CEL light was on and permanent code P0171 has been reported.
    2015 102k miles clean vehicle with decent dealer maintenance.
    I have cleaned and replaced MAF, but still the P0171 error eventually resurfaces. I have read many related threads shared here, but I am not sure regarding the next troubleshooting step. Today, I have used OBD Fusion Dashboard to monitor live data, but aside of the O2 Sensor 1 which did not move at all from 0.00 on 16 mile trip, I did not noticed any obvious issues. The auxiliary battery showed 13.7 to 14.4 while running and engine temp up to 190 F in hot sunny Arizona day.
    I know for sure the intake manifold has been recently replaced with a used part, and most probably the gaskets where reused, so I was thinking to explore replacing the O2 Sensor 1, swap PCV valve as preventive, cleanup the intake manifold, and replace all gaskets to make sure no vacuum leaks.

    I am trying to avoid start throwing parts at the problem and pray it fixes it.
    Cheers!
     
    #1 Marioaz, Jun 23, 2022
  2. bisco

    bisco cookie crumbler

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    why would the intake manifold be replaced on a 15 at 100k?
     
    #2 bisco, Jun 23, 2022
  3. Marioaz

    Marioaz Junior Member

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    The passenger to middle side has been damaged during a collision, as result the house inlet located in the lower side of the intake manifold where the vacuum tube connects it has been broken.
     
    #3 Marioaz, Jun 23, 2022
  4. bisco

    bisco cookie crumbler

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    #4 bisco, Jun 23, 2022
  5. Marioaz

    Marioaz Junior Member

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    Thanks much for the link! It includes an interesting thread for sure, but after numerous parts swaps most of the pervasive P0171 errors were resolved after fuel pump replacement. Do you know what is the normal range for O2 Sensor 1 while driving ? I do not see any input from that Sensor 1 monitoring with OBD Fusion, but O2 Sensor 2 is responding.
     
    #5 Marioaz, Jun 23, 2022
  6. bisco

    bisco cookie crumbler

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    idk. but i wonder if you need a more prius capable reader, like tech stream?

    have you watched the @NutzAboutBolts video?
     
    #6 bisco, Jun 23, 2022
  7. Marioaz

    Marioaz Junior Member

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    I have a cheap Techstream V 13.00 installed on a old Windows 7, but is crashing frequently when I attempt to run a test for specific component.
     
    #7 Marioaz, Jun 23, 2022
  8. mr_guy_mann

    mr_guy_mann Senior Member

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    The upstream air-fuel sensor on Toyotas actually uses a very small current reading to measure the exhaust gas mixture. Toyota engineers decided to have the ECM display a voltage value instead on a scantool. It should show something around 3.3V +/- 0.1V or so under steady throttle conditions. It goes lower towards 2.0V when rich, and higher (up to 5.0V with engine off) when lean.

    If you read 0 then something is up with your scantool. (If the AF sensor circuit was shorted to 0, then you would have different codes). I don't think I have that fault with OBD Fusion on my Gen2.

    The numbers to look at in ECM data are short term and long term fuel trim (on Bank 1). Good numbers are 0 +/- 5%. Check at idle, at moderate accel, and steady cruise. If numbers are high only at idle, that would suggest that there is a vacuum leak of some kind. Could be a bad intake manifold gasket, leaking hose to the evap purge valve, or something else related to the manifold replacement.

    Posted via the PriusChat mobile app.
     
    #8 mr_guy_mann, Jun 25, 2022
    Half Vast likes this.
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