2008 prius turns on by itself

welcome!
sounds like electrical gremlins. maybe caused by rodents, how many miles on her?

could you have left it on by mistake?

 

Or, no gremlins but several faults that have been ignored. Are there any error lights on to indicate that there are DTCs (diagnostic trouble codes) that can be read to focus on the problem areas, apart from the TPMS light mentioned?

Let me add my welcome to you too!

My guesses based on information presented but totally guesses in the absence of DTCs:

• CM (combination meter) has a failed capacitor. Does the dash go black (no speed, ODO, gear selection display, for example? A known fault, although not so common in 2008s; But a failed CM can make it hard to turn the car off. Always check the MFD (multifunction display) is black after you think the car should be off. If it isn't, press and hold the Power button for 5-10 secs until the MFD goes black.
• Check your inverter coolant reservoir when the car is in IG-ON or READY, for vigorous movement and/or put your hand on the inverter coolant circulation pump (below and behind the left (US driver's side) headlight for sounds and vibrations to indicate it is running. If this pump fails the car can run ok for quite a while until it gets hot or the demands of the inverter increase. After a time of cooling the car will run again. The fix is to purchase a replacement pump from the dealer ~$100 and replace it. You will also need to clamp off the hoses if the coolant is fresh otherwise, a gallon of Toyota SLL coolant is also required to refill after draining the old coolant.
• The seatbelt buckle most likely needs replacing to fix that issue but check the connector and wiring too. This problem can be fixed cheaply by getting a buckle from a pick-n-pull or wrecker's yard.
• One or more of the valve sensors that talk to the TPMS (tire pressure monitoring system) have failed. Replace all sensors that have failed the next time you are changing tires. Some tire shops have a good deal on replacing them or you can search online stores for Denso replacements. You will need to find someone with the right equipment to pair them to your car.

We can give better advice if you could give us any DTCs that were pulled from the car.

I hope that helps.


Prius Power Cycle States:

	To get to this state ...	Do this ...	then this ...	Notes
1	ACC	Foot OFF the brake	press start button once.
2	IG-ON	Foot OFF the brake	press start button twice.	(Once again if in ACC)
3	OFF	Foot OFF the brake	press start button a third time.	(Once again if in IG-ON or READY).
4
5	READY	Foot ON the brake	press start button once.

NOTES:

• Pressing the Start button once, with your foot ON the brake, from any of the above states (ACC, IG-ON, OFF), will start the car and make it READY to drive.
• When in READY and you press Power once (with your on OR off the brake), it will turn the car OFF.

 

You most likely left the car on when you got out the night before. It ran all night. Its near impossible for the car to start on its own.
And if the car ever shut off on the freeway it would have thrown lots of codes to tell you why that happened.

I can tell you for sure you are not setting the car alarm after you get out of the car. If you did and the car was still on it would not let you arm the car. And people who never set the alarm usually are the ones with repetitive dead 12 volt battery's. Because if a light is left on you cant arm it.

Lastly how old is the 12 volt battery? And do you know how to use a voltmeter? if you do we can load test the 12 volt battery.

 

I am following this feed. new to me '08 Prius 2 touring smart key with 130,000 miles. our car will not turn off, new fob batteries. dealer checked battery ok, no draws,faults. but dash light stays on....dealer is digging deeper into my pocket.

 

does the speedo work?

 

Ok car not turning off is very very common. Its a failure of whats called the Combo meter which is basically the dash led module.
Which is why Bisco asked is the speedo working.

The one that shows speed and mileage. Its a high failure rate item. And when it fails you cannot shut the car off.

You can buy a new module for about $100. Mine failed and only way to turn the car off was to disconnect the clickable 12 volt cable from the 12 volt battery.

If your DIY you can buy the Module for $100 and install it yourself. Go on youtube and search Prius Combo meter
and it will show you the steps to remove and replace that dash module on a G2 prius.

I bought my module from PC member Matt @ Texas Hybrids and 2 years later still running good. I installed it myself and I'm 65. But I'm a feisty 65 lol. With alot of tools.

 

That would be Matt @Texas Hybrid Batteries. Click on this link and then click Start a Conversation to get in contact.