Impending transmission failure maybe

It is almost certainly not a transmission failure.

A Prius transmission is (weirdly) much simpler than a traditional automatic transmission. There's no torque converter, so the fluid never carries the engine's torque to the wheels. This is the root of why there's no dipstick. Low fluid/poor fluid condition cannot cause the symptoms you're having.

Given the age of the car, a failing hybrid traction battery is a lot more likely.

This isn't a real diagnosis yet. First check the battery cooling fan under the left rear seat and make certain that the intake grill, blower and the tubing is free of dust and hair accumulation. There are youtubes that show how to do all of that yourself with only a screwdriver and a vacuum cleaner.

It's probably time to get a real diagnosis. This involves somebody connecting a scanner device or laptop and pulling the onboard diagnostic info. These cars are so sophisticated that trying to guess around the computer gets expensive fast. Whether you get your own setup or take it to a pro, that would be the next step after verifying that the battery cooling fan is clean and operating properly.

Toyota dealers certainly have the capability to do this, but they are a pretty expensive choice for service when you're dealing with a 10+ year old car. Lots of shops can handle hybrids now, and there's probably a garage closer to you than Toyota.

If it is the hybrid battery, it's going to be at least a couple thousand bucks to replace it. The good news is it renews the car for another 10 years or so- almost certainly cheaper than trading in for something else.

 

The hybrid control unit has authority to do whatever it wants. Sometimes it wants to spin the engine (without burning fuel) to reduce the battery state-of-charge. That can be in response to high battery temperatures or other conditions.

The computer can do this whether the car is moving or not. It doesn't really announce what it is doing, so it can look like it is just revving on its own without driving.

Other times it may choose to run the engine to raise the battery state-of-charge. Again, it's just the computer responding to a long list of sensory input and trying to do things to maximize the hybrid battery lifetime.

But when the battery gets old, the signals can be confusing and the corrective steps taken by the computer can be a lot more noticeable.

You can get tools that will give you a lot more information on this process, but in many cases you get too much information without the context to interpret and make good choices. I'll leave it to others to make specific recommendations.

You do not need a Toyota dealer for diagnosis, but they are by far the best supplier for the replacement batteries if the diagnosis reveals that one is needed.

 

Hybrid awareness has sort of snuck into every repair shop, though many don't advertise it.

But we're kind of at the point where a shop that won't work on a hybrid probably shouldn't be working on anything else made since the year 2000 either...

@bisco has been talking up a shop in Lawrence and may have more to add?

 

I wish. I only got that from someone here.
I have never found another, and the shops I’ve asked over the years aren’t interested.
Some won’t even do an oil change

 

It sounds more like a sensor glitch issue that cleared itself, when your shut-down and restarted. Like a very old computer that gets really slow or locks-up sometimes.
The way you check the ATF on that car is similar to checking the gear oil in a manual transmission. You remove the 10mm hex bolt located near the passenger facing drive axle. If the car is level, some ATF should trickle out. That's the proper level. IMHO, on a 10 year old car with however many miles on her, I'd go ahead and change it (~ 3.5 quarts) - no flushing required. Should be around $150, that's including parts and labor with OEM Toyota WS ATF - don't use anything else. There are plenty of YouTube videos on this procedure.

Good Luck.

 

And it's that way because this is much more like a manual transmission. There are gears physically turning each other. There are none of the fluid-dependent things (torque converter, bands, clutches) that are in an auto tranny.

In an auto, if your fluid gets low, things slip and don't transfer power, because the fluid pressure isn't there to clamp the bands or clutches or spin the torque converter vanes.

In a manual, if your gear oil gets low, the gears still carry power just fine—it's just at risk of wearing out or seizing up from lack of lubrication. The role of the oil in a Prius tranny is the same.

Usually, in a Prius, if the car starts behaving a certain way, but behaves normally again after you unhook the battery, that means it detected a problem and switched to protective fail-safe behavior. Behaving normally again after the power interruption doesn't mean the problem was a fluke, it just means you made the car forget there was one, so the protective behavior won't come back until the next time it's able to detect the problem.

 

The engine of my Prius C was revving higher than usual for a few weeks i think, or months, before displaying on the screen ''Check Hybrid System, pull on a safe place immediately'' or something like that, and it was the hybrid battery that had failed. The engine revs up to use the energy that the battery can't take anymore, and even when the car stops moving, the battery can't hold such a charge so the computer uses the ICE to discharge the battery further.