2005 Prius - after ECU replacement, car won't turn off... electrical problems

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Rebecca V, Mar 8, 2025 at 3:15 PM.

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  1. Rebecca V

    Rebecca V New Member

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    Huge thank you in advance for trying to help keep my beloved 2005 prius alive.

    My saga begins when on a cold day in Feb (but not that cold) my car wouldn't start. I thought somehow the battery died, and when I came back to try and jump it a couple days later it just turned on, no issue. Weird, but everything seemed fine for another couple weeks, including a visit to a shop to get an oil change and replace a belt - the shop never mentioned any intermittent starting issues so I assume the car behaved fine during that time.

    A day or two later, I got the large red caution triangle light on dash. My Prius is dead and stuck at a friend's house. I could turn the car on and start the engine but not put it in drive. I checked the codes and the car told me that it could not communicate with the car computer. I took a gamble on the integrity of the wiring and decided to replace the ECU with one that I bought on FlagshipOne. I disassembled my dashboard to reach the ECU, switched it out, and reassembled everything. In the process of course, I left all my doors open and lights on and drained my 12 V battery.

    Using a friend's external battery jumper thingy, I was able to turn the car on. I checked the OBD reader again and my computer code was cleared. I put the car in drive and moved forward 6 inches, very excited that it was working. Alas, I turned the car off again and couldn't restart it with that same external battery. There were again some weird dash lights that didn't seem consistent with anything I was doing, but I thought it was because my battery was very dead and wasn't getting consistent current from the external jumper battery (which itself would turn on and off). It was getting late, so I decided to go home and take my 12 V battery home with me and trickle charge it.

    When putting the 12 V battery back in, as soon as I connected the positive terminal, the car turned on, meaning all the dash lights not the engine. From this point on, I was not able to get the car to do anything - pushing the power button does nothing, taking the parking brake off doesn't make that "brake" light go off, the only dash light that will respond to anything is turning the headlights on and off. Certainly can't put it in drive, and unfortunately I didn't check the codes again that night. I unplugged the 12 V battery to turn the car off, and that is where I have left the car, still sitting on my friend's street.

    Has anyone seen something like this before? Any advice would be so appreciated!
     
    #1 Rebecca V, Mar 8, 2025 at 3:15 PM
  2. dolj

    dolj Senior Member

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    Anytime you disconnect the 12 V battery or it goes flat, you will lose any codes most ECUs set. The exception to that is some brake codes which are stored in non-volatile RAM so they survive a power out.

    It sounds like you could have more than one issue going on and it also sounds like the code reader cannot read all of the Prius computers.

    This thread reviews many phone-type scanner apps. Read the first page then skip to post #37 to read the cliff note version.

    One code (that does reset on a power out) is P0AA6 which seems to match the symptoms you mention.

    The key "feature" of P0AA6 is you need to reset the 12 V power every time it throws the P0AA6 so you can start the car and drive it. This buys you some time and might get you out of a sticky situation but should not be relied upon in the long term. You need to repair the car to eliminate the code.

    P0AA6 has 5 sub-codes (526, 611, 612, 613, and 614 – again a good scanner will display those) which will help narrow down which of the 4 sub-systems has the electrical voltage leak to ground which pops the P0AA6. You may find only 526 is set. That means you need to power cycle the car one more time. It does not matter if the car starts (i. e. goes READY) or not, you just need to power cycle the car. If you disconnect the power to the car before power cycling the car for a second time, it will clear the code and subcodes and you will have to start again.

    There is a Gen 1 non-scan tool procedure (for which you can search PriusChat) that will also narrow down the subsystem if you do not have an adequate scan tool.

    I hope that helps.
     
    #2 dolj, Mar 8, 2025 at 9:34 PM
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  3. Brian1954

    Brian1954 Senior Member

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    There are many (12 to 16) different ECU's in your Prius. Which ECU did you replace?

    What scanner are you using to read the trouble codes. Please be specific with manufacturer and model number.
     
    #3 Brian1954, Mar 8, 2025 at 10:30 PM
  4. Rebecca V

    Rebecca V New Member

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    Hi all and thanks for your attention - I replaced the ECM, the one that controls fuel injection etc. I used a Zurich ZR4 obd2 reader, which told me "U0100: lost communication with ECM/PCM 'A' ". I very well might need to use a different scanner, thank you for the advice. As-is, I am unable to power cycle the vehicle except by connecting/disconnecting the positive terminal of the battery.
     
    #4 Rebecca V, Mar 11, 2025 at 3:02 AM
  5. dolj

    dolj Senior Member

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    Have you checked fuses for possible loss of power to the ECM? I'm thinking the EFI fuse, maybe? Have you had your catalytic converter stolen recently?
     
    #5 dolj, Mar 11, 2025 at 10:04 PM
  6. Rebecca V

    Rebecca V New Member

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    I have not checked fuses - thank you I will look into that! My catalytic converter was stolen 3 years ago (sigh).
     
    #6 Rebecca V, Mar 12, 2025 at 2:41 AM
  7. MAX2

    MAX2 Active Member

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    If you have replaced the ECM, you can assume that one of the many connectors on or near the ECU is not snapped in place. It is very difficult to remove the ECU without disconnecting a bunch of connectors.
     
    #7 MAX2, Mar 12, 2025 at 5:11 AM
  8. Brian1954

    Brian1954 Senior Member

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    A U0100 code does not mean that the ECM is bad and needs to be replaced. It means that one of the other ECU's in the car can not communicate with the ECM. I do not think that the ECM was the problem.

    Your present scanner maybe is not reading all the trouble codes that your car has. If you want to purchase another scanner, look at the thread linked in post #2. The Autel AP200 is a very good choice. It is $60 +tax on Amazon.
     
    #8 Brian1954, Mar 12, 2025 at 9:40 AM
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