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P0171 lean code when cold outside?

Discussion in 'Gen 3 Prius Main Forum' started by koukilights, Jan 17, 2024.

  1. koukilights

    koukilights Junior Member

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    Over the last month we've had a few nights where it got into the negatives (Fahrenheit).

    I pull codes occasionally and was greeted with a pending lean bank 1 code when it was cold out.

    I assume the code doesn't trip again as the temp warms up and self clears, since the CEL hasn't actually come on.

    Is this just something that happens when it's cold with the denser air? I'd have assumed that the IAT sensor built into the MAF would detect and compensate for it? My Prius does have close to 300k on it, original MAF, maybe it's just getting tired?
     
    #1 koukilights, Jan 17, 2024
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  2. PriusCamper

    PriusCamper Senior Member

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    Yep... Your interpretation sounds about right to me...
     
    #2 PriusCamper, Jan 17, 2024
  3. ChapmanF

    ChapmanF Senior Member

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    It shouldn't be, and given that a MAF sensor is used, the IAT sensor isn't even needed to compensate for density (though the ECM may use the temperature for other things).

    A mass airflow sensor, as the name suggests, reacts inherently to the mass of the air flowing, not just its volume. It works by seeing how quickly the airflow carries heat away from the heated MAF sensor wires, and that effect is a function of the mass, not volume, flow.
     
    #3 ChapmanF, Jan 17, 2024
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  4. koukilights

    koukilights Junior Member

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    I guess my MAF element could just be dirty? I think I might have cleaned it 20-30k miles ago, but it is a 13 year old element now at this point. Maybe it's just not reading as accurately as it used to.
     
    #4 koukilights, Jan 17, 2024
  5. PriusCamper

    PriusCamper Senior Member

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    They do wear out. It's fairly common... I guess weighing how much air weighs is hard?
     
    #5 PriusCamper, Jan 17, 2024
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