Engine doesn't want to shut off sometimes

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by lumpy95, Sep 2, 2025 at 2:18 PM.

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  1. lumpy95

    lumpy95 Member

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    The last couple weeks, when I stop somewhere ( even when I put the car in the garage ) the motor just keeps running. I have waited up to 20 minutes for it to shut off before pushing the button to shut off the car and it keeps running until I shutoff the car. This is random, sometimes the car motor will shut off as it's supposed too Any idea what is keeping the motor running?
    Thanks for any responses.
     
    #1 lumpy95, Sep 2, 2025 at 2:18 PM
  2. Mendel Leisk

    Mendel Leisk MMX GEN III

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    In other words, Idle-Stop is not happening? Have you by chance got the "gear" in B (instead of D)? I think that'll keep the engine running. Maybe "PWR" mode too, not sure about that.

    Also, I've noticed on really hot days, especially with a lot of low-speed tooling around, the engine will keep running, maybe some hybrid battery protocol.
     
    #2 Mendel Leisk, Sep 2, 2025 at 3:14 PM
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  3. lumpy95

    lumpy95 Member

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    Thanks for the response Mendel.
    For sure not in "B".
    It's definitely hot here in the Mojave desert but I have never had this happen in the past hence my wondering why now.
    Before it started happening I had it in for service ( oil change, rotate tires ) and the mechanic changed the Air to Oxygen sensor but I doubt that would affect the motor not shutting off.
     
    #3 lumpy95, Sep 2, 2025 at 3:29 PM
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  4. lumpy95

    lumpy95 Member

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    Another thing I noticed is that the Hybrid battery gauge doesn't refill as fast as it used to so maybe your comment about the battery could be in play. Grasping at straws LOL.
     
    #4 lumpy95, Sep 2, 2025 at 3:38 PM
  5. rjparker

    rjparker Tu Humilde Sirviente

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    The engine will keep running in Ready IF the hv battery needs charge. Sometimes a really weak hybrid battery will not accept enough charge to let the engine shutoff at a stop.

    There is really no reason not to shutoff the engine when you park even if it's running.
     
    #5 rjparker, Sep 2, 2025 at 3:40 PM
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  6. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Try Dr Hybrid, check the hybrid battery condition?

    What's the miles?
     
    #6 Mendel Leisk, Sep 2, 2025 at 3:42 PM
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  7. lumpy95

    lumpy95 Member

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    Actually when it happened the last time the charge bars were almost to the top. I would understand if the Hybrid charge was low.
     
    #7 lumpy95, Sep 2, 2025 at 3:43 PM
  8. lumpy95

    lumpy95 Member

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    It's just under 32,000 miles ( obviously I don't drive much lol ).
     
    #8 lumpy95, Sep 2, 2025 at 3:45 PM
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  9. lumpy95

    lumpy95 Member

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    DrHybrid???
     
    #9 lumpy95, Sep 2, 2025 at 3:47 PM
  10. rjparker

    rjparker Tu Humilde Sirviente

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    A low capacity battery is really the most common reason. Other than that the engine will run if it needs to warm up but in the summer that is usually less than a minute. If it was winter, low coolant temperature might force the engine for heat. If you put the car in maintenance / inspection mode it would run continously, but that's not easy to do by accident.
     
    #10 rjparker, Sep 2, 2025 at 3:49 PM
  11. lumpy95

    lumpy95 Member

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    True but that wasn't the case for me.
     
    #11 lumpy95, Sep 2, 2025 at 3:51 PM
  12. rjparker

    rjparker Tu Humilde Sirviente

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    Bars do not show capacity. Next time it happens reduce the load by turning off the ac and lights.
     
    #12 rjparker, Sep 2, 2025 at 3:55 PM
    Last edited: Sep 2, 2025 at 6:06 PM
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  13. lumpy95

    lumpy95 Member

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    So your saying that the Hybrid battery may be going bad?
     
    #13 lumpy95, Sep 2, 2025 at 3:56 PM
  14. rjparker

    rjparker Tu Humilde Sirviente

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    Something is wrong with it. I would give it 98% likely. Could be corrosion on the bus bars (most old batteries have some) or a weak module(s). These battery modules are series connected. The weakest module or connection sets the bar. How many miles on the hv battery or was it already replaced with a "reconditioned" unit?
     
    #14 rjparker, Sep 2, 2025 at 4:02 PM
  15. lumpy95

    lumpy95 Member

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    Just under 32,000 miles so it's not the mileage. I bought it new and it's the original battery.
     
    #15 lumpy95, Sep 2, 2025 at 4:04 PM
  16. rjparker

    rjparker Tu Humilde Sirviente

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    Age and infrequent use is just as bad for a hybrid battery.
     
    #16 rjparker, Sep 2, 2025 at 4:06 PM
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  17. lumpy95

    lumpy95 Member

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    You could be right so I guess the answer is take it in to service and bend over. Thanks for your responses.
     
    #17 lumpy95, Sep 2, 2025 at 4:08 PM
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  18. PriusCamper

    PriusCamper Senior Member

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    Actually a brand new battery refills the slowest and a really old battery, which lacks capacity refills the fastest.

    One reason your engine would keep running would be if your battery is constantly getting drained down super low. Another reason would be if the heating or air conditioning is drawing lots of power. Lastly if your battery is fully charged it might spin the engine to dissipate the extra energy to ensure max lifespan of battery pack.

    Because you haven't mentioned anything I'd reboot all the computers to original factory settings by disconnecting the 12v battery for a few minutes. Then if the problem persists and your don't have any warning lights, I'd focus on a sensor sending bad data to the computers.
     
    #18 PriusCamper, Sep 2, 2025 at 4:38 PM
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  19. Brian1954

    Brian1954 Senior Member

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    I would not do anything until you get a trouble code.
     
    #19 Brian1954, Sep 2, 2025 at 5:24 PM
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  20. lumpy95

    lumpy95 Member

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    Thank you for the response.
    If I disconnect the 12v battery won't I have to reset everything ( windows etc. )? I don't have the knowledge to reset everything after unhooking the 12v.
    I do use Air conditioner in this heat and the battery gets low and the motor comes on to charge and the battery fills up when driving
    ( just not as fast as it used too ). All seems well except for randomly not shutting off the motor when stopping somewhere and I can see that the bars are way up on the battery ( not full but at least 3/4 full ).
     
    #20 lumpy95, Sep 2, 2025 at 5:29 PM
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