Acceleration & Slip Light

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by qbert1, Sep 14, 2025 at 1:55 PM.

  1. qbert1

    qbert1 Junior Member

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    Hello,
    My searching yielded nothing, so I guess I'll post.

    My 2009 has a problem that shows up randomly. Three times over the past month it started to fully accelerate without the gas pedal pressed. At the same time the "slip" light (the one that looks like the back of a car with squiggly lines behind it) came on - no other lights. Weather and road conditions were sunny and warm, all tires are practically new. Once it happened while pulling into a parking spot. The other two times at 35mph on a straight road.

    We cannot recreate this problem on command. Each time it was out of the blue and fortunately, no one was hurt. The brake pedal killed the acceleration and everything then went back to normal. I have an appointment with my trusted mechanic soon, but without the ability to recreate the problem, I don't know if the solution can be found.

    Any one else experience this?

    Does the VSC computer have the ability to accelerate the car if it thinks the the rear is slipping?

    Thinking of having him check the rear wheel speed sensors, yaw sensor, steering input sensor, abs pump (it's making noise - possibly more than it used to). Any other things to have him check?

    Thanks for any advice!
     
    #1 qbert1, Sep 14, 2025 at 1:55 PM
  2. bisco

    bisco cookie crumbler

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    hopefully, there are trouble codes in connection with the light. make sure your mech has a prius aware scanner
     
    #2 bisco, Sep 14, 2025 at 2:01 PM
  3. Tombukt2

    Tombukt2 Senior Member

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    Sounds like attraction control vehicle skid control computer issue possibly or it believes something else
     
    #3 Tombukt2, Sep 14, 2025 at 3:03 PM
  4. jdenenberg

    jdenenberg EE Professor

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    If the VSC activates, the engine power is shut down to prevent damage if the tires regain traction. So your VSC is not functioning as designed. Did you read any diagnostic codes? That's the necessary first step.

    JeffD
     
    #4 jdenenberg, Sep 14, 2025 at 3:33 PM
  5. ChapmanF

    ChapmanF Senior Member

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    There's a wider range of behavior possible from the traction control and VSC system than just to "shut down" engine power. That could be a closer description of the gen 1 Prius "traction control", which was really no more than a protection against over-revving an MG. In gen 2 and later, the system can do many more things, including supplying power to a wheel that's believed to have fallen below road speed.

    The advice to check for trouble codes is always sound, but it should not be too surprising if no codes are found. An ECU sets trouble codes when it detects and needs to report some functional problem. It does not set trouble codes for just doing what it is there to do. So, the antilock brake system engaging to keep wheels from locking up when braking, or traction control or VSC responding to a sensed traction loss or skid, will happen without trouble codes being set. Those are just examples of a system doing what it's there for. That can include when the system engages in response to some weird sensor input, if the input wasn't rejected as being weird.

    For diagnosing those situations, the "ABS History" page in Techstream is more useful.

    I have to be careful here to say that I know gen 3 has an "ABS History" page, but I don't know for sure if gen 2 does. The way it works in gen 3, anyway, it records all the relevant sensor inputs from the most recent four times the ABS or VSC or TRAC systems did what they are there for. So that's where you can look when you want to find out what was going on, when it wasn't something flagged as a fault and producing a trouble code.
     
    #5 ChapmanF, Sep 14, 2025 at 4:16 PM
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