Prius "transmission"

Do you mean that while driving it goes to neutral and you then have to shift it back to drive. Maybe just a bad shifter? If you hold it in drive does it then stay in drive? If that’s the problem it should be easy and pretty cheap fix.

What happened when you tried to shift into park?

 

"Slip" is a poor choice of words for this, as a "slipping transmission" means an entirely different thing.

In this case your "transmission" (the HSD) is doing exactly as told by the shift lever. Just the shift lever is going insane and telling it to do weird things. If you are in D or B and above 7mph if you (or the shifter gods) select R or P, it will go to N. We have seen this before on cars with heavily used shifters. A junkyard part is an easy swap, there is just a connector on the back. Pop some trim, unplug the old, plug in the new, bam new shifter.

Are you one of the people that shifts into N at every light, or between D and B thousands of times on a drive home? My coworker did that. His shifter also died. It has a finite life cycle like all switches, and usually it is used 3-4 times per drive. But if you're using it 300 times per drive, it will die a lot more frequently.

Also you can take this opportunity to "upgrade" your shift knob to something different. Since it does nothing but send codes, you can make it whatever you want. There is a flourishing aftermarket shifter knob market including just buttons for each gear versus the knob itself. If any of that appeals to you, now would be the time.

 

could be a bad shifter, how many miles on her?

 

You could check your electrical connections here also.
I am thinking this because it is exposed to the weather in the rust belt. I would check the electrical connector in the video for green wire oxidation ( bad connection).

 

any trouble codes?

 

Shifter is bad many posts about replacement. Frigid temps probaby got it.
Combo meter is next I’m afraid.

 

I don't know that any switches are in there ... just magnets and a couple Hall effect sensors that never touch them or move. I can't easily guess what would wear out in one, though from your post it seems like there's something that can.

Maybe some piece of metal finds its way in and sticks to one of the magnets, making it seem nearer the Neutral position than it is?

From the rest position to N is only one axis of movement, so just fooling the lateral sensor could look like a shift to N. To get spurious R or D, both sensors would have to be fooled.

I'd bet those button units use actual buttons (switches!) and might be even more at risk of wearing out, though I have no data.

-Chap