TAC, ABS, Data bus, and fail to start - where do I start troubleshooting

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Zedpi, Nov 20, 2021.

  1. Zedpi

    Zedpi Junior Member

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    2010 Prius
    Model:
    IV
    I wouldn’t usually ask for help this early in the process, but I don’t know where to start this time. It started with an abs/slip/brake/((!)) lights going on, then a few days later the triangle of death plus the “check hybrid system” message. And there’s a wirr from what sounds like the brake actuator every 2sec (its behind the coolant and brake fluid reservoir).
    The car was parked for about a week and I lent it to someone right before the abs lights. Currently it’ll start but immediately stop the engine, and I’m worried my Hybrid bat. will get too low if I don’t fix this soon.

    I have an obd II scanner, Dr.Prius and techstream available to plug in. Here’s a summary of the codes:
    P2118 - Throttle actuator control - range/performance problem
    P0A0F - Hybrid drive system - engine failed to start
    C1259,C1310,C1391 - from abs cu #2 (no msg)
    P0A0F - engine failed to start
    B2784 - entry/start ecu (no msg)
    U0155 - Instrumentation control module - no communication

    As you can see, I have a nice variety. I was hoping someone with some experience could find the connection here. Also unlikely to be related, I found a cat in my engine compartment a few days before this. (cat is doing good fyi)
     
  2. bisco

    bisco cookie crumbler

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    hmm... i wonder if the cat was after mice that chewed your wiring harness?
     
  3. ChapmanF

    ChapmanF Senior Member

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    It's not a one-connection-for-all-of-them picture. You've got some different things going on with the car.

    Two of the ABS codes (C1259 and C1310) are just echoes of the P0A0F code. (The brake ECU sets those two codes when it is notified about a code being set in the power management control ECU.) But the C1391 is about your brake pressure system unable to hold pressure, which is a separate real issue that you know you also have (because you also hear that pump frequently cycling.)

    The P0A0F code itself is a somewhat general code that is set by the power management control ECU when it has asked the ECM to start the engine and that didn't end up happening. The ECM has more information about what went wrong, but the power management control ECU pretty much only knows it didn't happen, so that's the P0A0F.

    (In fact, that code can have one of five "INF codes" going with it, which give a little bit more of a hint from the ECM about what kind of thing went wrong. But the real eyewitness details are in the ECM.)

    Since your only code from the ECM is the P2118, meaning a drop in the voltage supplying the throttle actuator, that would be my guess for the cause of the no-start. The voltage in question is supplied through the ETCS fuse to the +BM terminal of the ECM, where it is monitored. If it drops below 4 volts (that's a long, long way below 12), the ECM says "uhhh, nope" and sets the code, and cuts off the throttle control.

    B2784 is on a different topic, the connection between the Smart Key certification ECU and the key transponder built into the power switch. There are three pages in the repair manual to help troubleshoot that.

    U0155 is one of those network communication codes where you really would want to also know which ECU was saying it. It means "hey, I can't communicate with the combination meter", and it kind of matters who "I" is. (Who "I" ... am? There's no way of saying that right.) The power management control ECU, body ECU, certification ECU, and air conditioning ECU all communicate with the combination meter, so any one of them could report U0155 if it can't.

    (If you read the codes in something like Techstream, you see them all grouped according to which ECU reported them. Some other scan tools might only show what codes were reported and not who reported them, which kind of sucks.)

    I see one of the possibilities there is the certification ECU, so maybe there is some issue involving that ECU that would overlap with your B2784.

    What do you know about your aux battery condition? If the ECM really saw a voltage below 4 on the throttle circuit and if it isn't because of a dodgy fuse or connection, that could indicate the battery output got really severely low. That's the kind of thing that could also lead to communication error reports, so it's worth checking if you haven't yet. I'm afraid it will not supply an easy fix for your C1391 though.
     
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