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C1 259 DTC --- Hv system regenerative malfunction.

Discussion in 'Generation 1 Prius Discussion' started by 87Supra, Sep 7, 2021.

  1. 87Supra

    87Supra New Member

    Joined:
    Sep 7, 2021
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    Location:
    Washington DC Area
    Vehicle:
    2002 Prius
    Model:
    N/A
    Background info:
    I'm trying to help my uncle fix his 2002 Prius, which has been in the family since it was new and now has 250k on it. The symptoms it's having are loss of power and vibrations / knocking / clattering sounds under load. We started out with new plugs / coils / O2 sensors, but that didn't help. Dealer told him it needed a head gasket and wanted to put a new engine in the car. We ended up rebuilding the head with the engine in the car (found and fixed several engine issues while we were in there). After the rebuild, it did the exact same thing so we pulled the upper oil pan and inspected the lower end of the motor (yes, this can be done with the engine and drive unit installed in the car). Lower end seems fine. After the rebuild, compression is at 175-180psi on all cylinders on a cold engine. Everything seems fine with the engine IMO. So we read the codes (should have done that first!). Got a C1259, which seems to confirm my suspicion that it's the hybrid drive unit.

    Main Questions:
    If the problem is the hybrid drive unit, are the only options used or new (from Toyota)? Is there anyone that sells rebuilt / remanufactured hybrid drive units? Or that could repair the one we have?
    Are there any suggestions for things we could try before just yanking the whole unit and replacing it?
     
    #1 87Supra, Sep 7, 2021
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
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    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    While it is true that the fortune cookie for C1259 is something like "HV system regenerative malfunction", like most trouble code fortune cookies, that really doesn't tell you enough to be useful.

    What C1259 (which is a code set in the brake ECU) really means is that the HV ECU (which is kind of the orchestra conductor for the car) has detected some sort of issue under its own jurisdiction, and the HV ECU sent a message to the brake ECU saying "hey, I'm dealing with some sort of issue over here, please handle braking on your own, ok?", and the brake ECU responded with C1259 to say "yes, I got your message, and I'll do braking on my own."

    So any time you have a C1259, what it's telling you is that the trouble codes from the HV ECU are what you really need to look at to find out what that original issue is. And if you have got a scan tool that shows you the C1259 code from the brake ECU but does not show you the codes from the HV ECU that you really need, it will be hard to determine what the issue really is, until you have access to a scan tool that will tell you what the HV ECU trouble codes are. (There are enough of them, on the order of a couple hundred possibilities, that it doesn't make a lot of sense to try to guess.)
     
    #2 ChapmanF, Sep 7, 2021
    mroberds and JohnPrius3005 like this.
  3. 87Supra

    87Supra New Member

    Joined:
    Sep 7, 2021
    2
    0
    0
    Location:
    Washington DC Area
    Vehicle:
    2002 Prius
    Model:
    N/A
    Thanks! This is very helpful. I'm used to older cars that generally only have one ECU, so when I saw the code with a description that matched my expectations I didn't dig any further. I'll go back and see if the scan tool I have can access the HV ECU.
     
    #3 87Supra, Sep 7, 2021
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