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

Discussion in 'Gen 4 Prius Main Forum' started by swaldman, Jan 9, 2025.

  1. swaldman

    swaldman Junior Member

    Joined:
    Dec 10, 2019
    9
    2
    0
    Location:
    UK
    Vehicle:
    2018 Prius PHV
    Model:
    Excel
    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.
     
  2. davecook89t

    davecook89t Senior Member

    Joined:
    May 10, 2016
    1,073
    799
    0
    Location:
    Washington State, Florida
    Vehicle:
    2017 Prius
    Model:
    Four Touring
    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.
     
  3. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    2,232
    1,152
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    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?
     
  4. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

    Joined:
    Nov 1, 2016
    12,103
    11,529
    0
    Location:
    Central Virginia
    Vehicle:
    Other Hybrid
    Model:
    XLE
    Not all diagnostic tools read the Prius Hybrid specific codes. I think they are still stored historically if checked with a better tool.
     
  5. Mendel Leisk

    Mendel Leisk Senior Member

    Joined:
    Oct 17, 2010
    57,546
    39,711
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Prodigyplace likes this.
  6. swaldman

    swaldman Junior Member

    Joined:
    Dec 10, 2019
    9
    2
    0
    Location:
    UK
    Vehicle:
    2018 Prius PHV
    Model:
    Excel
    I determined that it was good by replacing it, to no avail :-/

    Now I have £100 less, and a spare battery that tests good!
     
  7. swaldman

    swaldman Junior Member

    Joined:
    Dec 10, 2019
    9
    2
    0
    Location:
    UK
    Vehicle:
    2018 Prius PHV
    Model:
    Excel
    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.
     
  8. swaldman

    swaldman Junior Member

    Joined:
    Dec 10, 2019
    9
    2
    0
    Location:
    UK
    Vehicle:
    2018 Prius PHV
    Model:
    Excel
    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"!
     
    vvillovv and Mendel Leisk like this.
  9. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    2,232
    1,152
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    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
    Last edited: Jan 9, 2025
  10. Prodigyplace

    Prodigyplace 2025 Camry XLE FWD

    Joined:
    Nov 1, 2016
    12,103
    11,529
    0
    Location:
    Central Virginia
    Vehicle:
    Other Hybrid
    Model:
    XLE
    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.
     
  11. BiomedO1

    BiomedO1 Senior Member

    Joined:
    Mar 27, 2021
    2,232
    1,152
    0
    Location:
    SacTown, Ca
    Vehicle:
    2021 Prius Prime
    Model:
    LE
    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.
     
    Prodigyplace likes this.
  12. davecook89t

    davecook89t Senior Member

    Joined:
    May 10, 2016
    1,073
    799
    0
    Location:
    Washington State, Florida
    Vehicle:
    2017 Prius
    Model:
    Four Touring
    If it's any comfort, here's the one incident I've had with my Gen 4 that I was reminded of when I read your post:

    Hybrid System Malfunction, Pre-Collision System Malfunction | PriusChat

    If you are not inclined to read the entire thread, let me get to the bottom line. I didn't take my car to the dealer for diagnosis of that one issue, and that's just as well, because they would have been able to find nothing, and in the almost 3 years since then, it hasn't recurred. Mind you, my problem only showed up once, although the day it happened, I drove it home and left it for a couple of days. It sounds like your issue popped up a couple of times, but all within a relatively short time period. I'll admit that for a few days I felt like I was walking on eggshells driving the car and not having a firm answer for what caused it has left me with less confidence in Toyota reliability.
     
  13. davecook89t

    davecook89t Senior Member

    Joined:
    May 10, 2016
    1,073
    799
    0
    Location:
    Washington State, Florida
    Vehicle:
    2017 Prius
    Model:
    Four Touring
    Actually, now I see my issue arose almost 4 years ago. Nothing since.
     
    Prodigyplace likes this.
  14. vvillovv

    vvillovv Senior Member

    Joined:
    Mar 19, 2013
    4,086
    1,396
    1
    Location:
    NY
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Plus
    If the dealer can't find anything wrong using techstream and their best diagnostics technician, my comments sure wouldn't be of much use.
    Maybe, get an odb2 bluetooth adapter and one of the phone diagnostic apps and see if they show anything obvious going on withe the car when it happens.

    Also, if you narrow the set of conditions, to any similarities you notice when it happens, that can be helpful too.
    If you narrow the set of conditions enough you may be able to figure out how to make it happen. Than techstream live data feature should show what the problem is.

    Some diagnostic apps also include live data recording, but usually not the set of safety sensors from what I've seen so far.
    Best of luck finding and getting fixed ....
     
    Brian1954 likes this.
  15. AzWxGuy

    AzWxGuy Weather Guy

    Joined:
    May 22, 2011
    1,014
    505
    0
    Location:
    Tucson, AZ
    Vehicle:
    2021 Prius
    Model:
    Limited
    Odd that a Toyota service department couldn't sort this out. Maybe try another dealer? I also wanted a clarification of the first error condition reported, ICS Malfunction, visit your dealer. I can find PCS but no ICS. Looking in section 7, When Trouble Arises, in the owner manual beginning on page 580 you can find most of those reported errors and what to do about them. Most of the error conditions sound like the result of a temporary low 12V battery problem. If you have a consistent low 12V problem it would be an error condition that if present would be reported and wasn't. Plus, the battery was replaced and the situation remains. Maybe have the new battery checked just to rule that out. Post #9 in this thread is very important for diagnosing intermittent low voltage situations which can cause a whole host of ECU malfunctions resulting in Christmas tree warning lights and messages. Something the Toyota service folks should have checked for. If you are a reasonably accomplished Prius DIY guy, you could have a go at the various electrical connections in and around the battery. Starting at the + and - posts on the battery and working outward until you run out of connections to check. Only check the + clamp on the battery, being careful not to short across to ground anywhere. Don't have any jewelry on your fingers either for this step. Then check all you can find from the - clamp outward. You are looking for rust or corrosion on any grounding connections to body panels or engine parts, etc. Even on clean connections do a cursory loosen and tighten to make sure the connection is good.
     
    davecook89t and Prodigyplace like this.