Ramifications of unknowingly driving with low coolant?

welcome!
the overflow reservoir does not determine if you were low. you did not do any damage.

before filling the reservoir, check your owners manual. there will be instructions as to the level based on a hot or cold engine.
all the best!

 

Gen 1 and Gen 2 had old-fashioned overflow reservoirs. Starting with Gen 3, there's a degas bottle instead, which is part of the circulating coolant system, and if the level is low there, that's real information.

Was the engine running or stopped for that "pre" photo?

I would keep an eye on it, see if the level holds. Your 60 month powertrain warranty should still be in effect, so don't be shy about having the dealer look at it.

 

I'm no prius expert but based on your level it appears there was always coolant in the radiator and the engine block more critically so I am pretty sure there was no harm or problem on driving it at that level any amount of distance. It would be interesting if it fell after adding more and could be indicative of a leak somewhere or a blown head gasket (very unlikely but covered under warranty anyway). Most likely it was just a bit low and by adding some it should resolve the issue. I would just take a good few pics with it full for verification and try to have anything repaired if possible within the 3 year, 36k mark or 5year mark if need be.


EDIT: After reading your post. I would just have Toyota service it since you fall under the 3 year 36 k mark and tell them that you smelled a burning smell and noticed a low coolant level and let them test and troubleshoot. You won't pay a dime for troubleshooting/repairs anyway since you fall under the warranty unless you got a hole in the radiator or something like that. If this were at 70k or 80k, I would buy the coolant but you should be completely covered under the 3 year warranty anyway

 

It does appear that it was just a bit low, but in a three year old car there shouldn't have been a way for that to happen.

 

Evaporation ?

And being AT the "low" mark means that it is somewhat low but still acceptable.

Nothing to worry about because of that alone.
Add up to the full or max. line while the engine is still warm and then check the next few days when the engine is COLD.

It is perfectly normal for a properly filled cooling system to be at or near the "low" mark when cold.

 

When our (new purchased) 2010 got to around the 4~5 year mark, both coolant reservoir levels had dropped from high to low lines. I topped them up and they’ve remained stable since.

Still, the smell OP noticed, and considering it’s under warranty: good to run it by the dealership.

 

From a sealed system?

 

Here we go again with these "sealed systems".
The cooling loop is NOT "sealed".

The pressure cap allows air flow in both directions.
Air out, if needed, as the pressure builds UP........and air back in as it cools down.

 

At least you're starting to think about the right things.

At what pressure does the pressure cap allow air out?

At what pressure is the cooling system designed to operate?

 

I have a similar issue on my 2017 Prius with close to 40K on it. Mine isn't below the low level line. Does Toyota top off the pink antifreeze during oil changes? I am going to take my Prius in for the 10k oil change within the next month.

 

unlikely but possible. i would check it before and after.

 

Hard to argue with that. Now, in a degas-bottle system like a Gen 3 and later Prius (and not a recovery-tank system like the earlier generations) ... what has to occur for that to happen?

 

<popcorn>

 

The entire system is still pressurized and the pressure cap still serves the same general function.
It is just physically located in a different place.
Actually the function is not exactly the same, as there is more "dead air" at the top of the bottle and any fluid that is lost is gone forever.
No automatic recovery but that isn't supposed to happen in "normal" operation.

 

You are getting very very warm now. What does the volume of that dead air at the top of the bottle tell you about what would have to happen to reach the cap's relief pressure?

Accurate except for the "still", right, because in the older design "the entire system" wasn't; the radiator cap was the demarcation between the circulating system, which was pressurized, and the recovery bottle and hose, which were not.

 

NOTHING.
Just stop, please.
This troll bait is getting old.