Quick way to determine a problem via ECU's

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Dennis Brown, Jul 15, 2026 at 1:46 PM.

  1. Dennis Brown

    Dennis Brown New Member

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    When a trouble light appears, it can be determined by shorting out the OBC II connector pins 4 & 13. Then the warning light will blink out numbers specifying the given problem. Of course a code reader will often be better but not always. Some codes are not read except by special readers.

    In my case, the TPMS system was blinking and my code readers did not say there was even a problem. When I shorted those two pins, all my trouble lights blinked at 0.25 seconds (normal - no trouble; see enclosed pics) except the tire pressure light. That blinked at its own rate but did not provide an error code. Immediately, I realized the ECU was the issue. Not the TPMS units in the tire nor the antenna transmitter in the rear of the car.

    I replaced the bad ECU unit in the console with a used one (very inexpensive.) I installed this new unit by opening the ECU plastic housing of the old unit (still in the dash.) Removed the old circuit board and slide in the new one into the original case. Far easier than trying to reach the case's lock down bolt. I notice this circuit board also used some caps that are not heat tolerant*. I reinstalled the plastic cover to the case.

    I then reinstalled the connector and the system worked fine.

    Aside: It had even recognized the new pressure sensors because I had first used a cheap $9 unit to reset the sensor in the tire and then used the reset button under the dash.

    As I mentioned, any given trouble light will blink but only the one with an issue will blink out a code. All others just blink at the 0.25 sec rate.


    *Might try fixing the old unit, later.
     

    Attached Files:

    #1 Dennis Brown, Jul 15, 2026 at 1:46 PM
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  2. bisco

    bisco cookie crumbler

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    congrats, thanks for the fyi!
     
    #2 bisco, Jul 15, 2026 at 1:49 PM
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  3. ChapmanF

    ChapmanF Senior Member

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    We have a page here that gives comprehensive information about that:

    Blink (a/k/a Flash) Codes – How to. | PriusChat

    Not all of the car's ECUs can do that; there are some that can only report trouble codes to a scan tool.

    Even for the ECUs that do participate in giving blink codes, some will use the same blink code for several different actual trouble codes, so a scan tool will give you better information. And many trouble codes are supplemented by three-digit "INF codes", which only a scan tool will show you.

    Nevertheless, when you have some warning lights on the dash and no scan tool handy, knowing what you can get with blink codes can make the difference between having some information and being completely in the dark.

    For the rest of the story about what you can and can't do with blink codes, there is the page linked above.
     
    #3 ChapmanF, Jul 15, 2026 at 2:11 PM
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  4. rjparker

    rjparker Tu Humilde Sirviente

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    Good reminder about blink codes. It is also possible to carry a $30 elm327 bluetooth obd2 interface with free apps like "Car Scanner" and "Dr Prius" to get more data, graphical trends and codes as necessary. Still not as good as a Launch or Autel based scanner device and app (some using bluetooth for less than $100).

    Ultimately some details, procedures or ecu programming need Techstream which has two versions available.

    I am curious what $9 device was able to reset the tpms id numbers in the new ecu using the old sensors. I know the "warning reset switch" under the dash sets a new tpms alarm point.

    IMG_2109.jpeg
     
    #4 rjparker, Jul 15, 2026 at 2:53 PM
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