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P0171 System Too Lean (Bank 1)

Discussion in 'Gen 2 Prius Technical Discussion' started by TMR-JWAP, Dec 24, 2019.

  1. TMR-JWAP

    TMR-JWAP Senior Member

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    Just wanted to throw this out here in case someone else runs into a P0171. This code triggers when there's too much air/not enough gas. (too much O2 in the exhaust) Although there can be many causes, a common one is merely a dirty mass air flow sensor.

    I recently purchased a 2005 with ~131k miles on it. I did pretty much everything I could think of to make sure it's current on all maintenance. I also installed a Gen 2 battery built with 2017 Gen 4 modules. I wanted to put a few tanks of gas through it prior to putting it up for sale, just to ensure there were no problems. I put about 1500 flawless miles on it and filled the tank one more time, planning to park it and go back to using my 2007.

    Eleven miles later, the CEL came on. First thing that pops into my mind is bad gas, since I just filled it up. Pull over and connect the laptop (never leave home without it) and it's a P0171. System Too Lean (Bank 1). Crap, can bad gas do that? That pump was supposed to be 10% ethanol, wonder if it was wrong? Anyway, car continued to run fine, so I reset the code. So much for parking the car. Halfway to work the next morning, code pops up again during a 1.5 mile slight uphill grade, clue number 2. Clear the code when I got to work. Codes again on the way home, on an uphill grade.

    Time to start checking the easy stuff. MAF sensor was first in line to be checked. Before and after photos. First photo you can see it has a nice black coating of grime over it. The coating acts as an insulator, causing the car to think there's less airflow than there really is. If it thinks there's less air, then it injects less fuel. So you end up with not enough gas for the amount of air that is really entering the engine...causing excess O2. Then the fuel trims get weird, triggering the CEL. The second photo is the MAF sensor removed from the intake housing and after a few shots of MAF cleaner. Back to being a nice amber color, with the wires visible again....500 miles later and the P0171 is still clear.

    IMG_5030.JPG IMG_5032.JPG
     
    #1 TMR-JWAP, Dec 24, 2019
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  2. spdracrm3

    spdracrm3 Member

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    That's actually the air temperature sensor you have pictured .if you tip it so the temp sensor is pointing at you and.look down the split tube in the middle with a flashlight you will see the resistor that is the MAF hotwire (it will most likely have the same fuzz on it) .clean it with a MAF cleaner and a Qtip being very gentile when touching it so as not to bend or break it and also blow it gently dry with compressed air like used on computers to remove any debris.should look like a clear glass bobbin on a wire when clean.

    SM-G960U ?
     
    #2 spdracrm3, Dec 25, 2019
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  3. ChapmanF

    ChapmanF Senior Member

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    +1 on the amber bulb being the temperature sensor, and the MAF being the wires you barely see if you squint into the long passageway.

    What I remember reading is to not touch those with anything, other than the stream of MAF cleaner from the spray nozzle.
     
    #3 ChapmanF, Dec 25, 2019
  4. TMR-JWAP

    TMR-JWAP Senior Member

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    Ya see....learn something every day. Prior to the Prius, the only time I ever had to mess with a MAF device was my 1989 Mustang. That had the element exposed. I just assumed designs had changed over the years, with theory remaining the same. I'll check it again to be certain, but I'd imagine after removing it and the spray I gave it, nothing escaped the wrath of the can...…..
     
    #4 TMR-JWAP, Dec 25, 2019
  5. TMR-JWAP

    TMR-JWAP Senior Member

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    Ahhhh...this looks much more familiar...just happened to have the one I removed from my spare parts car sitting on a shelf in the garage.
    Although the code has stayed clear, I definitely think I'll pull it out and spray it again tomorrow, just to be sure. I appreciate the input.

    IMG_5121.JPG
     
    #5 TMR-JWAP, Dec 25, 2019
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  6. Robert 2010 prius

    Robert 2010 prius Junior Member

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    I had the same issue with this P0171 code
    Change the intake manifold gasket ,air flow meter , at the end of the day it was the fuel pump . Change it yesterday and the cel light went off
     
    #6 Robert 2010 prius, Nov 3, 2020
  7. dolj

    dolj Senior Member

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    It is not such an easy fix in the NA Gen 2. In order to change the fuel pump, you need to change the entire fuel tank.
     
    #7 dolj, Nov 4, 2020
  8. gsu13

    gsu13 Junior Member

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    Thought I would post my 2+ year experience with a P0171 Too Lean (Bank 1) code on a 2005 with about 250,000 miles. I used a lot of info on this forum, youtube, then a trusted mechanic. Maybe this will help someone else

    A couple years ago the check engine light was on for this same reason - Too Lean. I tried cleaning the MAF and throttle body - no luck. Then a new MAF - nope, then a new PCV - nope. Then premium fuel and Techron fuel additive. That seemed to keep the check engine light off enough to pass emissions back then. But it came back soon after and I just left it as my mileage was still about 47 mpg

    Now 2 years later this time, it wouldn't stay off for more than 15 miles and didn't think I would pass emissions again. Tried again to clean the MAF, clean the throttle body and then premium fuel and Techron. Nothing worked (and don't know if it even really helped last time). So I was considering changing the upstream air/fuel mixture sensor and if that didn't work, then considering a new fuel pump. But my mechanic said it is weird the upstream sensor didn't have its own code besides just a P0171 Too Lean code, so I thought I would buy another MAF one more time.

    The check engine light has been off for almost 100 miles and I just passed emissions inspection. And fuel mileage seems to be up to 52 mpg versus maybe 47. I don't know if I ever got the MAF fully cleaned (the wires down inside vs the more visible amber bulb) but seems like the new Denso MAF is working so probably just going to leave it



    A couple years ago the check engine light was on for this same reason - Too Lean. I tried cleaning the MAF and throttle body, then a new MAF and new PCV, then premium fuel and Techron fuel additive. That seemed to keep the check engine light off enough to pass emissions back then. 2 years later, this time, it wouldn't stay off for more than 15 miles so that's why I contacted you after trying to clean the MAF again, clean the throttle body and then premium fuel and Techron.

    But after you said that the upstream air/fuel mixture sensor could be replaced, but it is weird it didn't have its own code besides just a P0171 Too Lean code, I thought I would buy another MAF one more time. The check engine light has been off for almost 100 miles and I just passed emissions inspection. And fuel mileage seems to be up to 52 mpg versus maybe 47.
     
    #8 gsu13, Apr 17, 2021
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