Battery ECU reporting erratic module voltages

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by NiallS, Aug 6, 2026 at 10:08 PM.

  1. NiallS

    NiallS Junior Member

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    Hey folks,

    Our Prius recently threw four hybrid battery fault codes (P3000, P3030, P3012 and P3013)

    Initially suspected modules 2 & 3 were failing based on P3012 & P3013. Then realized the ECU was reporting wildly erratic and physically impossible voltages for those modules. See attached chart showing modules 1 and 4 holding steady, while module 2 swings 10V to -15V, and module 3 swings +10V to 25V.

    Curious if anyone's seen this kind of fault before? My guess is the harness is corroded at the connector or module and/or the battery ECU is failing. Planning to open it up tomorrow to inspect.

    Thanks,

    Niall
     

    Attached Files:

    #1 NiallS, Aug 6, 2026 at 10:08 PM
    Last edited: Aug 6, 2026 at 10:18 PM
  2. dolj

    dolj Senior Member

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    I think you're on the right track, and I would be leaning toward corrosion in the sensing wires of the No 2 Frame Wire causing a break in one or more of the wires. I would replace the No 2 Frame Wire if this was the case, and if the corrosion reached the battery ECU plug, I'd clean the ECU pins or replace the ECU if the pins were too damaged.
     
    #2 dolj, Aug 7, 2026 at 6:53 AM
    Last edited: Aug 7, 2026 at 7:01 AM
    Brian1954 and NiallS like this.
  3. NiallS

    NiallS Junior Member

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    [mention]dolj [/mention] Thanks so much, good to know I am probably on the right track.

    ( If any members near Chevy Chase MD happen to have a used sensing harness or battery ECU for sale please shoot me a DM or reply here!)


    iPhone ?
     
    #3 NiallS, Aug 7, 2026 at 9:49 AM
  4. dolj

    dolj Senior Member

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    A new replacement harness is available from ACENBAY for $85: Toyota Prius Hybrid Battery Wire Harness Assembly.

    Used ECUs are readily available from wreckers or eBay.
     
    #4 dolj, Aug 7, 2026 at 9:52 AM
    Last edited: Aug 7, 2026 at 9:58 AM
  5. dolj

    dolj Senior Member

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    @NiallS, try using the @ in front of a user's to 'mention'.
     
    #5 dolj, Aug 7, 2026 at 9:55 AM
  6. gdanner

    gdanner Member

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    Faulty sensing harnesses are the number one cause of totally out-of-spec voltage measurements from apps like Dr. Prius. I've even seen "negative" values reported. The results are random and extremely erratic. After replacing the wiring harness I still observed some intermittent module voltages and codes, but not as extreme as from a faulty wiring harness. This turned out to be from corrosion on some of the busbars and loose or damaged hex nuts which tighten the busbars against the threaded posts on the battery modules.

    Several vendors sell sensing harnesses at ebay and Amazon. I've purchased two harnesses from this vendor. They both worked perfectly. Price was reasonable: $26.59 USD with free shipping.

    https://www.ebay.com/itm/388680417248

    Next, it is necessary to disassemble the battery ECU and examine the interior portion of the orange voltage sensing connector. It gets corroded on the inside as well as on the outside. I used contact cleaner and a toothbrush. I also found that dilute vinegar solution helps loosen up the greenish blue corrosion deposits. I've successfully cleaned two battery ECU units in 2 different cars. Sometimes the orange sensing connector gets badly damaged (broken pins for example). Or the PC board itself is damaged. In this case I recommend replacing the ECU. As mentioned above it is easy to obtain used battery ECU modules.

    Also I look closely at the busbars and their connection to the battery modules. If they are corroded I recommend replacing the entire set of busbars with this kit (also sold at ebay):

    https://www.ebay.com/itm/385429404100
     
    #6 gdanner, Aug 10, 2026 at 1:33 PM
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