Car revving high as I brake and at idle

1. The throttle assembly may have being clogged with lots of carbon deposits, especially on the butterfly thingy.

You could remove and service that with carb cleaner, or remove the air intake hose, and spray the carb cleaner into the throttle assembly, as the engine is idling (trying putting the engine on inspection mode or something, to allow for extended idling).

2. There maybe a vacuum leak somewhere, that's allowing unmetered air into the engine.
That might account for the high revving. The ECU is trying to compensate for a lean situation.

3. The throttle assembly might be faulty (I mean, the electronic small motor that controls idling).

Keep us posted pls

 

The engine will rev if the traction battery is full to keep from overcharging the battery

 

FWIW, the one instance I've noticed engine rev up, and it's quite reliable, is after having the 12 volt battery disconnected. For the next maybe 2 start up, it'll rev up for a few seconds, like it's going through some calibration.

 

welcome!
hybrid battery might be going south, how many miles on her?

i would risk the drive to the dealer

 

Any check engine light on the dashboard? If no, check the spark plugs. If worn excessively, they'd cause poor mpg issues.