Roommate's 2017 low coolant, unsure how to proceed.

Warm up the car then fill to the full line. If you don't have Toyota coolant, use distilled water. It may just be low from 7 years of neglect. Monitor for the next few months.
FWIW, 2016 - early 2017's had exhaust gas heat exchanger issues that may be covered under TSB stealth recall/repair.

Good Luck.....

 

TL ; DR
You 'HAVE' coolant in the....reservoir? overflow? tank.....so this is 'probably' a non-emergency.



Longer Answer......
a 2017 - WHAT?
If it's a Prius then I would advise your RM to look in the glovebox and see if perhaps the previous owner left an owner's manual.

If not, and IF your RM isn't very bright then you can help him out by pointing out that the manuals are available on line.
Toyota Manuals and Warranties | Toyota Owners

As much as I bag on Toyota for their predatory corporate culture, they actually make it fairly easy for norms to maintain the cars that they sell.
Mostly?
It's a "keep it between the lines (or dots) thing." for fluids.
They also have a VERY EASY TO FOLLOW warranty and maintenance guide that nearly everybody does not follow.
-ESPECIALLY their dealership maintenance departments!!!
That's one part of the corporate culture thing that I was alluding to earlier.

I'm not going to pretend that the level in the pictures is normal, abby, or anything in between because this is just one pixel in a fairly large picture.

Have you ever added fluid?
If the vehicle has 73xxx miles then what did it look like at 70,000?
65,000?

How many owners?
How many accidents?
Why did you have the hood open and how often does this happen?
Lots of things to consider here!!!!

ALSO, the the factory manual isn't the end-all, be-all of car maintenance!!!
ONE example is that they recommend 10,000 mile oil change intervals....AND 1,000 mile oil check intervals.

That's ANOTHER part of the corporate culture thing that I was alluding to earlier.

 

Yep and I'd advise him to get a mechanic to do a "drain and fill" on all the coolant...there's two systems but use the same coolant and it's a simple job. Tell him to NOT pay for a "flush"...that's just a trick for mechanics to charge you a lot more.
At 73,000 miles, it's also overdue for a transmission drain and fill....should be done every 50,000 miles or 5-years, whatever comes first.
Toyota's have a history of being able to take neglect but, in the past 10-years or so, they've seem to lost some of that ability, which is sad.

 

Editorial:

Toyota specs 100k miles or 10 years for first coolant change, so he's not overdue.

With our 2010, about 5~6 years after (new) purchase, both reservoirs had dropped to or a bit below the low lines. I got a 4 liter bottle of the spec'd coolant and topped them up. Nieither budged from then on, when I changed both circuits, at the 10 year mark. I think a slight drop like that is normal.

That said, early model year 4th gens are prone to coolant leaks in the Exhaust Heat Recovery system, so he should keep a close eye on those levels.

 

since the gen4's has a "pass-through" cooling system - as long as he's able to keep a good amount of coolant in that bottle - he should be OK. Tell him to monitor, before every startup. Right now, he's between low and that lower hose, if coolant levels get down to that hose level - the coolant pump will begin to suck air into the cooling system - that's a whole set of other problems.
IMHO: If your friend doesn't care, he can just blame Toyota for this. Hopefully he's has a huge bank account, cause the mechanic will surely 'bend-him-over' for his lack of knowledge and inability to do basic checks and take basic care of minor issues before they blossom into large repair expenses.

Just my 2-cents.....

 

The system is basically like any other internal combustion engine's coolant system (for all particle purposes). The level is supposed to be maintained between the "hi" and "lo" marks. While it is true the maintenance schedule does say to change it at 10 years/100,000 miles, it also says (at every 6 months/5000 miles) "Inspect engine and inverter ... coolant level/condition/freezing point. Your dealer may recommend services ... based on inspection results". So basically, the coolants should be tested quite often to determine its condition and changed when the inspection results indicate the it is starting to deteriorate. Since you will probably be buying some coolant anyway to bring it up to the correct level why not change it, or at least test it before adding the CORRECT coolant (NOT distilled water as that will only dilute the coolant and raise the freezing point of the old coolant (that may or may not be in good shape). And BTW do not mix coolants (brands or types). Get the correct Toyota coolant. Assuming it is the original coolant, the levels are consistent with a neglected coolant system (very few dealerships actually maintain the systems by even adding coolant, much less actually testing the coolant).

 

What's the other overflow look like? Prius or Prius Prime? Looks about 3 cups low. Look at the owners manual for amount of coolant the engine side takes. While you're at it it's good to also check how much coolant the inverter side takes for your model.
I'd assume it's the same amount for the Gen 4 Prius and Prime, but assumptions are the root of all screwups, good to make sure yourself.

 

I would leave it at that. If $20~ out-of-pocket is too much him, maybe time he learned the hard way. That's the price of a cheap bottle of wine fur chrisakes.

 

@Mendel Leisk , he'll learn alright - IT'S TOYOTA'S FAULT FOR MAKING A CRAPPY CAR. Time to ask mommy & daddy to bail me out again....

 

Last time I bought was 2020, and IIRC it was $24 CDN, for a 4 liter bottle of 55% mix (how it comes up here). Through dealership parts department.

‘Course when I check just now on Amazon.ca it’s $69.99 CDN.

addendum: my price, through dealership parts department in October 2020, was $22.95 CDN, per attached:

 

TL ; DR
As mentioned by @BiomedO1 the coolant system will be fine as long as their is fluid in the tank and it's mostly between the low and full lines.
It's important to check every now and then, but this isn't something to be anal about.
- - - -

OK.
So it sounds like your roomie isn't into the car maintenance thing, but it sounds like he's been spending money for maintenance.
That's actually not a bad thing if he can afford it.
As much as I rag on Toyotas, the G4 Prius (2016-2022) is probably one of the better of the generations, especially for reliability.
Keep ALL of the fluids topped off and pay some passing attention to the other items in the Warranty and Maintenance guide that I sent a link to and you are almost dead-bang guaranteed to have a 250,000 mile, not many issues car - especially if you're reading all of the maintenance tips in this forum.

As mentioned by @BiomedO1 the coolant system will be fine as long as there is fluid in the tank and it's mostly between the low and full lines.
IIRC The tank that you're looking at is a pressurized coolant reservoir. The clue for me is cap that warns you against removing it when the engine is hot.
It also means that you don't want to fill it very much past the 'full' line, but save the laser level for hanging pictures.
It's not that critical. It's a coolant reservoir, which means it captures the coolant that overflows when the engine is warm and return it to the radiator when it cools - so as long as there's fluid in this tank your engine will always have the right amount of cooling fluid in the engine with NO AIR.
Air is bad when it gets into the coolant loop.

The reason that others were commenting on your roomie is because he is perhaps spending much much more money on car maintenance than he needs to because he isn't spending about 10 minutes a month doing some very basic maintenance checks to avoid some fairly expensive repairs.
Think of it like this:
Maintenance is like a medical check up.
Repairs are like surgery, which is more expensive and....not all doctors are ethical or equally talented, and not all outcomes are great.

If you buy a $20,000 car and your 'maintenance plan' is to throw the key fob on the desk at a dealer's maintenance department and hope that they will be gentle with you?
You deserve to get ribbed good naturedly about it.

Good Luck.


Last thought....
CONSIDER changing to 5,000 mile oil changes if you're not already doing so.
The oil is just fine after 10,000 miles but that is for lubrication.
The reason that oil turns black after 5,000 miles is from dirt and other contaminants.
This is controversial in an age where putting oil into plastic bottles makes the polar bears cry but as you read up on Prius maintenance you will begin to understand that this is something to think about.

It's beneficial for people that don't check fluids to change the oil more often because there is the possibility that the dealer will check it after they change it and you will not ride around for an additional 5,000 miles low on oil.

 

I did shake that habit for a few years: bought bulk Toyota oil from dealership, bringing my own container, then walking down a trail to Mr Lube (local quick-oil change place) with a similar container with the drained oil.

No one-use, oil-contam’d containers in the equation. Then dealership started jacking the price of Toyota bulk oil, more than slightly, going from around $3.50 CDN per liter, to over $10, then refusing to sell to me anymore.

so now I’m buying 5 qts like everyone else, but it’s disappointing.

 

First off; I'd like to apologize to you and your roomie. Re-reading that post, it was a bit harsh.

You and your roomie don't seem to have much knowledge or experience with cars. Most of us are very knowledgeable and passionate about cars. This is a chance to nip a potential issue in the bud, very easily and cheaply - if you went with the distilled water approach. While other people in this forum say, don't do it - mixing in a couple of cups of distilled water isn't going to foul-up the coolant mixture enough to cause it to turn into a block of ice.
The benefit here is that you can determine if it's leaking or simple abuse that cause that issue. If it holds steady after topping-off, it was abuse and you don't need a mechanic to look at it or test it - keeping valuable greenbacks in your pocket.
If it is leaking, well you'll have to take it to a mechanic. Hopefully, they are honest and very knowledgeable. People here had paid $2.5K+ for a head-gasket repair when the issue was the EGHE. Another $2K repair. I can assure you that the mechanic isn't going to admit he was wrong and misdiagnose it. He'll just tell you that was broken too. You don't pay him, he'll take your car.

sorry again....

 

EGHE I think stands for Exhaust Gas Heat Exchange, also referred to as EHR (Exhaust Heat Recovery). I'm not sure even the latter is legit in Toyota parlance. Anyway:

The Exhaust Heat Recovery system salvages heat that would otherwise go out the tail pipe, by passing the exhaust gas through a radiator that has coolant flow, accelerating coolant warm up when the car's just cold-started. And with fourth generation the system's been prone to spring a coolant leak.

Toyota has acknowledged the issue, but is dragging their feet, regarding educating dealerships, ensuring sufficient production of EHR replacement component, and just generally "pulling their weight". Too, dealerships doing the repair purportedly get paid less if Toyota is the client, versus a retailer customer. Also doesn't help, that Toyota combined the EHR system with the catalytic converter component, made it a $ingle part.

So, both the dealerships and Toyota have a motive to "not notice" the acknowledgement of fault, have the car owners shoulder the cost. And maybe suffer misdiagnosis as well, though: protracted coolant loss often does lead to head gasket failure.