2008 Prius Goes into ACC mode by itself

welcome!
could be a problem in the fob slot electronics, low fob battery, defective 12v, loose 12v wiring, time to start eliminating the low hanging fruit.

 

Here's what I'd try first (free and easy to do): check the brake light switch operation. With the car on, rest you foot on the brake pedal and see if the lights activate. If not, gently press the pedal until it does. The adjustable switch may be out of spec. so the car may not be recognizing the brake is being pressed during the power down phase and going to ACC instead when the power button is pressed.

 

yes, the positive and negative connections at the battery, and also where the negative connects to the body. inspect for corrosion as well.

and to fotomot's point above, if you have your foot on the brake when pulling the fob out, that may be an issue.

 

Only one key fob? Does the issue exist with others?

 

Be sure you have more than one key fob. It is much easier and less expensive to add one if you have a working fob.
It usually requires more expense & dealer reseeding if there is no good fob.
Some people have found the negative connection to the body loosens too.

 

Amazon, ebay, etc., approx. $100 for a NEW fob is considered a decent deal. Dealers are usually MUCH more. Do not buy a used one; they won't pair. If you lose the last remaining FOB or it stops working, you'll have a MUCH more expensive problem on your hands (dealer/locksmith).

With a new FOB, you pair it yourself to the old FOB/car by doing the chicken dance: How to Program a New Key Fob for Full SKS Functionality | PriusChat

 

@Ryansb13-
Did the loose connection fix your random starting car?
My 2008 Prius has 206481 miles and has decided to start turning on by itself.
It also does the no dash lights most of the time when I go to start car.
Sometimes it’s like it has no battery power and will do nothing. Sometimes when you shut the doors it will then have battery power again and inside lights will light up.
If the dash does not light up it does not like to turn off! Cruise control does not work either when the dash does not come on. I did find a thread that says to hold down the power button without your foot on the break until car cuts off. Sometimes it works but most of the time the car motor stops but everything else is still powered up. I then have to double mash the power button to power everything down.

Love my car but it’s driving me crazy!
Help any suggestions on all this craziness going on?

 

In my understanding, when the combination meter stops working right (a known issue involving a capacitor), it stops doing its needed steps in the dance of powering off the car. So you may find that fixing the combination meter issue will fix the powering-off issue.

 

+1 to ChapmanF.
Your symptoms certainly sound like a faulty CM (combination meter- speedometer- instrument cluster). You might want to read the "sticky-ed" at the very top of the Gen2 Care, Maintenance, and Troubleshooting forum.

Posted via the PriusChat mobile app.

 

Ever get a resolution to this? There are a bunch of these posts and the only thing I ever see is tried tightening battery connection and no follow up. (Hey maybe that means it worked).

 

Why don't you try it? Then you can post back with a resolution to all the threads you've resurrected.