Small motor or pump running after car is shut down

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by [email protected], Sep 18, 2025 at 3:59 AM.

  1. Racerman15@comcast.net

    [email protected] Junior Member

    Joined:
    Jan 24, 2010
    32
    10
    0
    Location:
    San Francisco
    Vehicle:
    2020 Prius
    Model:
    LE
    2020 Prius, 50k miles. Gas mileage 52+, power is very good. No operating issues But - several times in the past few weeks I have come into the garage and heard something running from under the rear of the car. It is a small electric motor sound. If I turn on the car in the usual way the sound stops and stays off once the car is turned off. Two times I know that the noise was not present immediately after the car was turned off - but I observed the noise a couple hours later, and not sure when it started or how long it had been running. One time I let it run to see if it would turn off by itself. I checked it after 45 min and it was still going. I am thinking that it might be the hybrid cooling fan, fuel pump, ??? I was hoping for some ideas before I started taking things apart.
     
    #1 [email protected], Sep 18, 2025 at 3:59 AM
  2. FuelMiser

    FuelMiser Senior Member

    Joined:
    Jun 29, 2008
    1,349
    928
    0
    Location:
    Monument, CO
    Vehicle:
    2016 Prius
    Model:
    Touring
    It's normal.
     
    #2 FuelMiser, Sep 18, 2025 at 8:49 AM
    BiomedO1 likes this.
  3. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    3,089
    1,636
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    Your battery would be DEAD, if something keep running after the car is shut down. How would you find an intermittent problem like that by ripping it apart???

    The car does readiness testing at certain intervals and if it see the FOB in your pocket - it's going to fire everything up. It's like a pre-flight check list before startup.
    Drop you FOBs in a Fairiday bag or throw them in your refrigerator and see if the car does the same when you look-in on it. I know they used to build houses over there with chicken wire mesh to hold up the plaster. That also blocks RF signals, so when you open up a door, a weak FOB signal becomes a stronger FOB signal - indicating to the car that your approaching and likely going to drive her.

    Hope this clears things up.....
     
    #3 BiomedO1, Sep 18, 2025 at 10:01 AM
    Last edited: Sep 18, 2025 at 10:31 AM
  4. Robert21PrimeLTD

    Robert21PrimeLTD Member

    Joined:
    Jul 30, 2023
    79
    39
    0
    Location:
    Nashville
    Vehicle:
    2021 Prius Prime
    Model:
    Limited
    it’s probably the fuel evaporation leakage check. happens approx. 5 hours after using the vehicle. check your manual.
     
    #4 Robert21PrimeLTD, Sep 18, 2025 at 10:27 AM
    Leadfoot J. McCoalroller and Iron Man Prius like this.
  5. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    27,002
    17,716
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    That's the only odd part of your story. In the operation sequence of the 3rd-gen evap test, phases A, B, D, and E add to 8 minutes 10 seconds and phase C can be up to 15 minutes, so 23 minutes 10 seconds is about the maximum duration of the test in gen 3. The pump isn't on in phase A, so 22:10 should be the maximum run of the pump.

    [​IMG]

    Admittedly, I don't have a gen 4 repair manual open right now to look up the corresponding diagram. Maybe it does take longer in gen 4. But my first reaction is 45 minutes seems long, and I would want to recheck my observations.
     
    #5 ChapmanF, Sep 18, 2025 at 7:32 PM
    BiomedO1 likes this.
  6. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    3,089
    1,636
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    What do you mean by this? "One time I let it run to see if it would turn off by itself." This would infer that you know what's running and can manually turn it OFF???
    Do you mean that you left the car in READY mode and didn't turn it OFF?
    Did you sit there for 45 minutes listening to the weird noises or you walked away from it and came back to check on it after 45 minutes????

    Inquiring minds need to know.......
     
    #6 BiomedO1, Sep 18, 2025 at 7:52 PM
  7. Racerman15@comcast.net

    [email protected] Junior Member

    Joined:
    Jan 24, 2010
    32
    10
    0
    Location:
    San Francisco
    Vehicle:
    2020 Prius
    Model:
    LE
    "turn off by itself" - i casually observed the small motor running, about 5 hours after i had parked the car. I had learned by then that I could stop the small motor by turning the car on and then back off, but I did not do that in this instance, jut to see if the small motor would go off by itself. It had not stopped 45 min later but was not still going when I can out in the morning.
    This has been very unscientific and it does look like it is the self test for the fuel vapor test. I will look into that further.
    Thanks to everyone for the very speedy input! I will check codes and also be accurate recording the times it runs.
     
    #7 [email protected], Sep 19, 2025 at 2:40 AM
    BiomedO1 likes this.
  8. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    3,089
    1,636
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    Yes; turning the car ON and OFF will reset the self diagnostic timers. Also keep in mind that when the car 'sees' the FOB approaching - it's going to prep the car for start-up. Pressurizing the ABS braking system is one of them - that's why sometimes you feel the brake pedal stiffen during start-up.

    Hope this helps.....
     
    #8 BiomedO1, Sep 19, 2025 at 12:30 PM
(You must log in or sign up to post here.)