Won't shift into gear, occasionally

check the 12 volt health with a free load test at an auto parts store. how old is it?

 

That's no guarantee.

Still, I don't think it's the 12v. I sometimes had that on my 2005 Prius. It only seemed to happen if I was in a hurry. If I slowed down just a fraction, pressing deliberately on the brake pedal and on the Start button, before a kind of deliberate move of the shifter joystick, it worked every time. I'm not saying to act like the sloth in Zootopia, just slow down a little bit.

 

Here is my tranny horror story, it might help someone, same issue, wouldnt go into gear, eventually we were stranded.
I will cut the long long story short, after lots of messing around I came to the conclusion that the power train control module was bad, got one off ebay and was then told the dealer had to program it, then was told, no, turn the car on with a battery charger and leave it and it will program itself, and that worked, for a few days, then the problem returned. I went over the schematics and found that the main 12 volts for this module, which is mounted next to the engine ecu under the glove box, comes from a fuse under the hood, when i pulled that fuse it disintegrated into powder, fairfax county road salt destroyed the fuse. replaced it, no more shifting issues. car now has 300k on it still going.

 

Thats even a better reason to test it. If it fails, probably still under free replacement warranty unless it’s a really cheap one.