Intermittent fault affecting all electronic safety systems : where to look?

Discussion in 'Gen 4 Prius Main Forum' started by swaldman, Jan 9, 2025 at 10:46 AM.

  1. swaldman

    swaldman Junior Member

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    Hi folks,
    Last time I posted here was 2019, shortly after I bought my Prius (PHEV/Prime, 2018 I think - the last year that didn't have a rear middle seat). All has been great for that time, but recently upon turning the car on I got a christmas tree of warning lights and a sequence of warning messages:

    • ICS Malfunction, visit your dealer
    • Braking Power Low, Stop in a safe place
    • Braking Power Low, Visit your Dealer
    • Blind Spot Monitor malfunction, visit your dealer
    • Lane Departure Alert malfunction, visit your dealer
    • Pre-Crash safety malfunction, visit your dealer
    • Antilock braking system malfunction, visit your dealer
    It was an especially cold day so my first thought was the 12V battery. But that wasn't it.
    I called out a breakdown service, and they couldn't see any error codes. They told me to take it to Toyota.
    I took it to a Toyota dealer, and.... it stopped misbehaving. All looked fine to them, and they couldn't see any codes either. They told me it was safe to drive.

    Then a few hours up the road, it did it again. And then fixed itself. And then did it again....

    So, TLDR: I seem to have an intermittent fault. My nearest dealer is a costly ferry ride away,and the local garages have waiting lists. I will put it on a waiting list, but my question is: Are people familiar with this problem? If it all looks fine when it arrives at a mechanic, is there anything I should suggest that they have a look at?

    Thanks.
     
    #1 swaldman, Jan 9, 2025 at 10:46 AM
  2. davecook89t

    davecook89t Senior Member

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    Has the 12V battery ever been replaced? How did you determine it was good? Apart from rodents possibly chewing on some of your wiring, your initial take is likely correct. It is the 12V.
     
    #2 davecook89t, Jan 9, 2025 at 11:06 AM
  3. BiomedO1

    BiomedO1 Senior Member

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    Is this ONLY happening when your try to start the car or does these trouble lamps light up while your driving?
    When was the last time you changed the 12vdc battery and who did it?
     
    #3 BiomedO1, Jan 9, 2025 at 11:27 AM
  4. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

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    Not all diagnostic tools read the Prius Hybrid specific codes. I think they are still stored historically if checked with a better tool.
     
    #4 Prodigyplace, Jan 9, 2025 at 11:28 AM
  5. Mendel Leisk

    Mendel Leisk Witness Leader

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    #5 Mendel Leisk, Jan 9, 2025 at 11:46 AM
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  6. swaldman

    swaldman Junior Member

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    I determined that it was good by replacing it, to no avail :-/

    Now I have £100 less, and a spare battery that tests good!
     
    #6 swaldman, Jan 9, 2025 at 1:20 PM
  7. swaldman

    swaldman Junior Member

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    No - it first happened at startup, but now it will come and go while driving.
    12V battery was changed about an hour after it first happened - that was my first guess! It was changed by somebody from Halfords.
     
    #7 swaldman, Jan 9, 2025 at 1:21 PM
  8. swaldman

    swaldman Junior Member

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    Don't think there's much in common - I don't think that one is intermittent, and that one is about different (worse) errors. With mine, while it disables all the modern safety features, I don't see anything that makes me worse off than driving a car from the 90s ;-)

    But I do have a video, which is helpful. When the dealer told me "there's nothing wrong" I showed it to them and they went "oh" and went and looked for another half hour. Then changed "there's nothing wrong" to "we can't find anything wrong"!
     
    #8 swaldman, Jan 9, 2025 at 1:25 PM
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  9. BiomedO1

    BiomedO1 Senior Member

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    I'm thinking loose or corroded power connections and/or bad wiring somewhere. If there's no codes, on an OEM Toyota certified scanner, your systems/ECUs must think it's a key-off situation, therefore no error/trouble codes are generated.
    I'd put on some gloves and start tugging/wiggling/twisting connectors, in hopes of duplicating the issue. The 12vdc system isn't going to hurt you, it may spark - but it's harmless. Stay away from any of the orange cable stuff - that's the high voltage stuff that'll hurt you. I'd start at the 12vdc battery connections.

    Hope this helps and Good Luck....

    Follow the black ground cable from the 12vdc battery. Make sure all connections are tight and corrosion free. There's a lot of plastic in these newer cars, older metal cars use the car's chassis as ground. Now you need ground wires to major components to ensure a good current return, so if one of those wires are loose - that's your intermittent trouble warning.
     
    #9 BiomedO1, Jan 9, 2025 at 1:49 PM
    Last edited: Jan 9, 2025 at 2:17 PM
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  10. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

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    in the 90's you could actually look out the rear windows. Since the mid 2000s Toyota has designed their cars with higher fenders and smaller side back windows along with the wide pillars to the rear window. The 4th Gen & earlier Prius had the extra lower window, if you could keep it clean. I do not recall if the Prime had that.
     
    #10 Prodigyplace, Jan 9, 2025 at 3:18 PM
  11. BiomedO1

    BiomedO1 Senior Member

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    Yes, that 3" piece of glass is there.:whistle: It's kinda useless if your short, the cargo shade tips down so you can see out of it and I always tip my rear seat head rest down - for better rear visibility. I removed my old Prius C rear head rest for the same reason. It bounced around in my trunk, in case I had passengers.
    I believe it was eliminated on gen5s.
     
    #11 BiomedO1, Jan 9, 2025 at 5:12 PM
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