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The Dreaded P0171

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by joeprius2018sea, Jun 20, 2022.

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  1. joeprius2018sea

    joeprius2018sea Junior Member

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    I think the next step is to replace the fuel pump...just want to make sure.

    Car Info: 2012 Prius 4 175k

    Never took it to a dealer, just had a local mechanic take a look.

    Things I've tried:
    Clean MAF
    Replaced MAF (Denso)
    Smoke test for leaks - None
    Replaced O2 Sensor (Denso)
    Spark plugs done at 100k

    Any ideas, thoughts much appreciated it.
     
  2. PriusCamper

    PriusCamper Senior Member

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    MAF Wires and plug look ok? Maybe clean and inspect?

    How many miles and what other kind of maintenance over its lifespan?
     
  3. joeprius2018sea

    joeprius2018sea Junior Member

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    yes, MAF plug looks fine. I cleaned it first, no change, then I replaced it.

    175k miles, I've done all schedule maintenance on the Prius.

    I've had the P0171 code for almost 3 months now. At first, I would clear the code, it didn't come back for weeks. But now, if I clear it, it comes back after 20-30 miles.
     
  4. Half Vast

    Half Vast Junior Member

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    Have you looked at fuel delivery? Fuel pump, injectors? Found an interesting video which diagnoses it in a Prius. Could be something to check.

     
  5. joeprius2018sea

    joeprius2018sea Junior Member

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    Thanks for the link, looks like he came to the same conclusion, replace fuel pump.
     
  6. Half Vast

    Half Vast Junior Member

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    If you have the ability, I'd perform the same test to see the waveform of the pump. That'll help eliminate the possibility of just blindly throwing parts (and money) at the problem. Good luck with this and post back what the fix is.
     
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  7. PriusCamper

    PriusCamper Senior Member

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    Sure would be cool if we had some images or videos of the healthy wave form of various parts that fail on a Prius... That'd be an awesome resource. And for that matter, what's the least expensive way to get a scope that can do this?
     
  8. ChapmanF

    ChapmanF Senior Member

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    There are pumps that fancy in a Prius, but I don't think the fuel pump is one of them. I think it just has power and ground; it's on or it's off.
     
  9. Half Vast

    Half Vast Junior Member

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    I have a Hantek 6022BE and find it quite useful. I use it with the same laptop that I have Techstream on. I got mine from Amazon.
     
  10. Half Vast

    Half Vast Junior Member

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    Found this video which breaks down testing on a lean running Prius engine. Thought it was quite informative.

     
  11. Half Vast

    Half Vast Junior Member

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    True in one aspect. I believe that since this is a brushless motor, it is commanded "on/off" by pulse code modulation. It is switching DC currents to the windings thereby causing the rotor to turn due to the expanding/collapsing magnetic fields. The waveform, taken from a current probe attached to one of the power leads on the pump gives an indication of the expand/collapse of the magnetic field within the motor.

    While watching the first video, I spotted the current probe attached to the fuel pump and knew exactly what the Tech was looking at.
     
  12. ChapmanF

    ChapmanF Senior Member

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    I don't think so:

    copn.png

    Simple relay that applies 12 volts across the motor.

    There are other things in the car (engine water pump, inverter pump) that have separate connections for 12 V power and a pulse train input to set the speed. Not this one.

    It probably does have an interesting current waveform, as any motor would, just from the commutation happening inside it (whether that is done electronically or with brushes). That said, you're probably on your own to interpret that waveform, as it isn't one that Toyota mentions in their troubleshooting. They just have you check the motor's resistance, and whether it pumps when 12 volts are applied.

    fp.png

    The fact that a resistance is specified suggests to me that it is a conventional motor with brushes. An electronically-commutated one typically doesn't look much like a simple resistance across the terminals (you'd be trying to measure the 'resistance' of some active electronic circuit).
     
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  13. Mendel Leisk

    Mendel Leisk Senior Member

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    Here's some Repair Manual excerpts. If they're pointing to other pages let me know; I'll try to find them.
     
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  14. joeprius2018sea

    joeprius2018sea Junior Member

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    Thanks for all the info!
     
  15. Half Vast

    Half Vast Junior Member

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    I stand corrected.

    Without a manual nor any true experience with these cars (I've only had mine since March) I had to make some assumptions. I always enjoy reading your posts on the various topics here as I always learn something. I appreciate the detailed explanations.

    Still, I am curious as to the vast difference in the waveform's in the one video. Time for a bit more reading and learning.

    Thanks for posting these. I need to find a manual somewhere for my 3rd Gen.
     
  16. joeprius2018sea

    joeprius2018sea Junior Member

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    FYI - I got the fuel pump replaced. Drove 100 miles so far and the light haven't come back on. MPG is better now too.

    Got the parts from a dealer in CA.

    Fuel pump - 7702047081 $400
    O-ring - 7716906060 $15
     
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  17. Half Vast

    Half Vast Junior Member

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    Awesome! Glad to hear you got it running again! Thanks for posting the follow-up. It'll help others down the road.
     
  18. Prius RT

    Prius RT Junior Member

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    So did replacing fuel pump solve p0171 permanently?
     
  19. BlizzardBuzzard

    BlizzardBuzzard New Member

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    There's so much quality info in this post, I figured I'd reply here rather than start a new one. I assume I have a vacuum leak, but given the above info, maybe there's an issue on the fuel side leading to P0171. I did the EGR service and spark plugs 1000 miles ago and now it's throwing this code on the second sub-freezing morning at idle. The most likely scenario is that I didn't put a hose clamp back properly and there's a vacuum leak. It's 7 weeks to get an appointment. Any other ideas to try first? STFT is better at high RPM, worse at or around idle.
     

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