Gen 2 Prius: Water in trunk -> U0293 code. Radiator fans run at max speed, ICE starts for 15s

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by MutiVinas, Jun 13, 2026 at 10:43 AM.

  1. MutiVinas

    MutiVinas New Member

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    Hi everyone,

    I am facing a very frustrating issue with my Gen 2 Prius (2007) and really need some expert advice.

    Background:

    A few days ago, I discovered that the trunk was severely flooded (water accumulated in the spare tire well and heavily leaked into the left side quarter panel area). The car locked up, threw a P/S warning on the dash, and the radiator/cooling fans under the hood (front of the car) started running at maximum speed continuously as soon as the ignition was turned on.

    What we have done so far:

    1. We completely drained the water and thoroughly dried out all the wiring harnesses and connectors visible in the trunk using a heat gun/hairdryer.

    2. We removed the trim panels to access the Battery ECU. We inspected the unit, and physically, the module and its pins appear to be completely dry and clean inside.

    3. We even swapped the Battery ECU with another known-good working unit, but the exact same symptoms persist.

    4. We followed the reset procedure by disconnecting the 12V negative terminal and removing the orange HV service plug for over 20 minutes to clear the deep memory, ensuring the service plug was properly slid down and locked back into place.

    Current Symptoms:

    When we clear the codes via OBD (using Car Scanner) and press the Start button, the ICE (Internal Combustion Engine) successfully fires up. However, it runs extremely rough with violent shaking and vibrations. After exactly 10 to 15 seconds, the car automatically stalls/shuts down. The front radiator fans continue to blast at maximum speed. Once it stalls, the scanner brings back only one single, stubborn active code:

    U0293 (Lost communication with hybrid powertrain control module).

    My Questions:

    Since the front radiator fans are blasting at max speed, it feels like the main ECM or HV ECU is in a fail-safe mode due to the lost communication. Since the Battery ECU itself seems fine (tested with two units), could the water in the trunk have caused a short circuit that blew a specific integration relay up front, or damaged the CAN-Bus line entirely? What should be our next step to diagnose this U0293 loop?

    Any guidance on which specific wire pins, fuses, relays, or hidden connectors to check next would be highly appreciated.

    Thank you so much in advance!
     
    #1 MutiVinas, Jun 13, 2026 at 10:43 AM
  2. Tombukt2

    Tombukt2 Senior Member

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    Test 12V battery . Check the white plug above 12v battery look at it carefully . All that flooding renders a battery showing volts with nothing behind the volts switch in a load . Boom nothing .
     
    #2 Tombukt2, Jun 13, 2026 at 12:24 PM
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  3. PriusCamper

    PriusCamper Senior Member

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    Not related to your problem: The water damage you described in Gen2 is common and is likely coming from roof seams you can see under the black roof trip strips as well as areas you can see when rear hatch is up. Clean them and add some silicone or equivalent in the cracks.

    What the problem is: The engine is failing to start. The 15 seconds is just the electric motor spinning the engine trying to start it. Keep in mind if you spin an engine that won't start too many times you drain your hybrid battery down and recharging it without a functional engine is a whole other hassle we can walk you through.

    Solution: If I were you I'd swap out the ECM after you thoroughly inspected and haven't found any damaged wires. Then I'd probably look at engine temperature sensors to confirm they're working normally. And If that doesn't work I'd look at the spark plugs for clues, maybe use a borescope to see what's going on inside there. Clean and inspect Mass Air Flow sensor and throttle body and all this is assuming you followed @Tombukt2 advice on 12v.
     
    #3 PriusCamper, Jun 13, 2026 at 1:22 PM
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