Coolant System and Hybrid Invertor Coolant Flush

It's not really a "flush', but more of a drain and refill.

I guess you can get some sort of acidity / pH tester on Amazon to check the coolant. I have not tried this personally yet:

Here is a good overview of the drain and refill process for both the engine and inverter. Also his channel is great for other DIY stuff:

 

In USA, first engine coolant change is 100k miles, and 50k thereafter. Inverter coolant change is 150k miles, 50k thereafter.

assuming the car’s only had the 100k engine coolant change, at 189k you’re long overdue for both.

 

As was stated, it is a drain and refill.
And it's NOT a stealership. They do not steal anything. The customer approves the work.
Just because it cost more at a dealership doesn't mean they are stealing.
Independent shops rip people off more often that dealerships.

You do NOT need to go into maintainence mode. If you fill the engine coolant VERY SLOWLY
and a cup or 2 at a time, and wait, you're here it draining to the bottom. And if you REALLY
listen, you can hear the air being pushed out.
When you pour it slowly and little by little, it allows the coolant to push the air out and not allow it
to creat air pockets. And you want to fill it to the "B" line. Yes, it will take longer to fill, but you
won't have issues with overheating, and waste fuel trying to get the air pockets out.

After filling, start the Prius, and let the engine run until it shuts off. Check for leaks. If nothing
is leaking, check the level and fill as needed to just above the seam/full line.
Then drive it around the block, keep the engine running so it will get up to temperature and pressure.
Then park it and check the level. It will likely be the same level. But wait a few hours, or overnight.
Then top it off if needed.
I've done it like this 4 or 5 times and haven't had an issue.
And it's the way I've been doing for all cars since the mid 80's....

The inverter drain is fill is simpliar, and quicker. You don't have the air pocket issues.