CEL - P0301, P030027 all the sudden with shaking

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by Pyler, Aug 9, 2025 at 10:30 PM.

  1. Pyler

    Pyler Junior Member

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    Drove 20+ miles in 95f+ heat of LA weather. Not sure if that could have caused this. Basically got off the freeway with the car rattling on acceleration that slowly got worse and eventually got a CEL so had to leave the car close to home.

    Could the heat have caused a head gasket failure or could it just be a spark plug? Trying to determine first course of action.

    Errors are showing:
    P030027 - Random/Multiple Cylinder Misfire Detected: Signal Rate of Change Above
    Threshold

    P030100 - Cylinder 1 Misfire Detected
     

    Attached Files:

    #1 Pyler, Aug 9, 2025 at 10:30 PM
  2. bisco

    bisco cookie crumbler

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    how is the coolant level? have you dealt with a leaking exhaust coolant heat exchanger?
    you'll need a boroscope to check the head gasket.
    if the plugs are original, and you have less than 120k, they aren't the likely cause, but they could be fouled from poor combustion
     
    #2 bisco, Aug 10, 2025 at 1:33 PM
  3. Pyler

    Pyler Junior Member

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    Haven't checked the coolant level, but it gets routine service and has never had any cooling issues. Prob the easiest thing to try first is just swapping cylinder 1 with another one to see if the error changes?

    Not sure if plugs are original or not, I did have a broken engine cylinder spring a few years ago that was tough to find and I'm not sure if they ended up also changing spark plugs at the same time or not. I bought it in 2019 with 70k miles.
     
    #3 Pyler, Aug 10, 2025 at 2:13 PM
    bisco likes this.
  4. Danno5060

    Danno5060 Active Member

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    Maybe check that you didn't get some bad gasoline. Was this not too long after you filled up somewhere new?

    Also, the 20K mile oil service Toyota recommends isn't the best idea. Even if you're using extended mileage engine oils. One of the issues is that the variable valve timing is controlled using engine oil. Overused engine oil makes for variable valve timing not working correctly. Change the engine oil (I use just regular Mobile 1, but if you have a preferred brand, use that - definitely synthetic though).
     
    #4 Danno5060, Aug 10, 2025 at 3:54 PM
    Last edited: Aug 10, 2025 at 4:01 PM
  5. jdenenberg

    jdenenberg EE Professor

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    As Bisco said, check your coolant level. 2016-2017 Prii have a history of developing coolant leaks in the catalytic heat exchanger. This can cause oveheatin the engine and costly engine problems. There is a low cost fix (bypassing the heat exchanger) discussed on PriusChat.

    JeffD
     
    #5 jdenenberg, Aug 10, 2025 at 4:16 PM
    Iron Man Prius likes this.
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