P0121 and Hybrid Battery Warning

Discussion in 'Prius c Care, Maintenance and Troubleshooting' started by Ferdman, Dec 8, 2025 at 10:06 PM.

  1. Ferdman

    Ferdman Junior Member

    Joined:
    Apr 22, 2024
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    Location:
    southern California
    Vehicle:
    2014 Prius c
    Model:
    One
    Hi all. My first post.

    I have a 2014 Prius C with 250K miles. It was given to me 40K miles ago. Head gasket went bad and got a replacement engine rather than have the head decked and new gasket. Shop said the block was warped too or something to that effect. Replacement engine had 70K miles on it and didn't cost much more than the head gasket job. It runs great, but occasionally gets a warning about the hybrid battery and to stop the car as well the P1021 code. Once I am able to clear the P1021 code as I keep a code reader in the car, I am able to drive off normally. The P1021 code is for the throttle position sensor for the engine, but what does that have to do with the hybrid battery/drive? The throttle position sensor is on the throttle body and really isn't replaceable unless changing out the throttle body itself. I'm going to try to clean out the throttle body or replace it to see if that fixes the issue, but how is that related to the hybrid system? Any help would be appreciated. Thanks.
     
    #1 Ferdman, Dec 8, 2025 at 10:06 PM
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
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    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    You may want to edit your post (or ask the moderator to do so, if your edit button is gone already) to say P0121. It's right in the title, but typo'd in the post itself, which could make it harder for others to find with a search.

    This is probably a good place to link to the post about which codes are throttle sensor and which ones are pedal sensor, because that's a recurring source of confusion in the Prius. You got it right here, though; your code is about the throttle sensor.

    The fail-safe response for the ETCS codes is supposed to be:

    [​IMG]​

    If you keep driving after that happens, you're using only battery power to move the car, and there is only enough in the battery for a quite short distance at low speed before you'll get a warning about that.
     

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    #2 ChapmanF, Dec 9, 2025 at 8:41 AM
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