Codes P0A94, P0AA4, P0AA1 for 2005 prius. Cant figure out whats wrong.

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Ferazzi331, Jun 11, 2020.

  1. Ferazzi331

    Ferazzi331 New Member

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    Hey guys, just joined this forum as i am having issues with my 05 Prius. I will start off from the beginning. I just bought the car last week to flip it. p/o stated it needed 2 cells and he had it checked by the stealership. well sure enough.

    took apart the battery and all of the cells tested at 7.85-7.86 except for two cells which tested at 6.5. Replaced both with ones that were at 7.55. i didn’t bother to balance the battery since i figured they will charge and balance over time. drove the car and the battery on trip info held charge great. couple min in the Red triangle along with the other lights came back on. no worries its probably panicking since two cells are lower. drove the car around a few miles and then parked it.

    today, i took of the battery case and took off the bus bars to remove the corrosion off of them. tested all cells again and all were still around 7.85 but the two had now charged up to 7.80. cool. its working. put the case back on and went for a drive. it was driving perfect. did multiple 0-60 pulls with no issues while monitoring temps and codes and performing full battery tests through Dr. Prius. decided to take the car on the highway to just charge it up more.

    On the way back home, the ABS, TCS, and VSC lights came on. and then the car just died and went into neutral. the infotainment system went black so the 12v battery seemed sus. eventually only thing that would happen when i went to start is the CEL would come on. Anyways. jumped the car from terminals upfront and it was ready with no issues. no lights and drove perfect all the way home until i reached the driveway where it threw the red triangle and CEL and died. Restarted it immediately and just parked it on the driveway. Checked codes and they are P0A94, P0AA4, and P0AA1.

    I know nothing about hybrids and this is my first prius. one thing i noted is that after the jump the 12v was steady at 13.40v, just checked right now with the car off. its not starting again and 12v battery is at 8.6V. seems like something is draining power or maybe a bad 12v battery? not sure why those codes would come up then. maybe just freaked the electrical system out because of low voltage like my german cars tend to do lol. any help/ tips would be greatly appreciated. it has 117k miles on it and was parked for a year i believe when the owners were in Africa. so maybe the 12v battery is the issue but it should not be draining that much with the car off.
     
  2. sam spade 2

    sam spade 2 Senior Member

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    Yes. Or maybe your new battery never was charged fully.

    At 13.4 V when running, it won't charge much; that is about what the "float" voltage should be.

    The "other" end of the negative battery cable is often a problem too; loose or corroded.

    I suggest that you start by disconnecting one side of the battery, fully charge it and then let it sit overnight.
    If it is an AGM battery, it should still be at 12.8 or more.
     
  3. AzusaPrius

    AzusaPrius Member

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  4. TMR-JWAP

    TMR-JWAP Senior Member

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    P0A94= DC/DC converter

    The DC/DC Converter on the Prius is like the alternator on a normal car. Once the car is in ready mode, it takes power from the HV system and converts it to low voltage to power the 12v system, along with charging the 12v battery.

    When not in ready, the 12v system should be at battery voltage (typically 12.3-12.8 volts)
    When the DC/DC converter is functioning properly, that voltage jumps to ~13.7-14.1 volts.
    See? Just like how an alternator powers the 12v system on a regular car.

    With the P0A94 (DC/DC Converter Performance)...something is happening (or has happened) that is causing the converter to code out.

    There's a couple possibilities
    1. The 12v battery is in such bad shape that the converter is unable to keep up with the charge rate being drawn.
    2. The converter has actually failed, which will prevent the 12v battery from charging, along with being unable to power the 12v systems in the car.
     
  5. sam spade 2

    sam spade 2 Senior Member

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    His measured 13.4 is a bit low.
    Likely he does have a problem with the converter.
     
  6. TMR-JWAP

    TMR-JWAP Senior Member

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    Also, of major significance, I didn't realize it at first, but the other 2 codes, the P0AA1 and P0AA4 are saying the main battery relay circuits are stuck closed. That could be bad and needs to be investigated.
    Having zero experience with hybrids, perhaps the work you did on the battery missed something? or maybe it's just all related to the 12v battery being DOA?
     
    #6 TMR-JWAP, Jun 12, 2020
    Last edited: Jun 12, 2020
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