Overheat and coolant loss from reservoir, how to tell if water pump or head gasket

If it’s your headgasket, you can remove the spark plugs and put a borescope down there to see if the pistons top is wet or not. Also, you should have a leak in the coolant reservoir if it’s the HG

 

Get a gas test on the coolant, if there is combustion gasses in the coolant, it's a head gasket, if not, put cardboard down under the engine and check in the morning just where the coolant leak is dripping from .... you might need to remove the plastic tray under the engine (if it still has one) and redo the drip test a few times to isolate where the problem leak is .... the water pump leak will appear at the fan belt end of the motor, easy enough to replace, unlike the gen 2, Google Search

 

That test doesn't work on the Prius because it's the coolant that leaks into the cylinders.
Unless it's REALLY bad, but by then you won't need that test, you'll KNOW it's the head gasket
and likely a warped head.

There is no pulley on the Prius coolant pump.

 

I agree with the dealership that you need to replace the engine water pump before you drive the car again. An OEM pump is preferred. Do not buy a cheap aftermarket pump on Amazon or eBay. They do not last very long, and you will end up having to replace it again.

If you have a bad head gasket, you would get misfires (rattling) during cold startup of the engine. I do not think that you presently have a head gasket issue. The P0117 code indicates that the engine severely overheated. This can lead to the head gasket going bad 6 to 12 months from now.

So, replace the water pump ASAP and consider a replacement car in 6 to 12 months if you do not want to deal with a bad head gasket.

 

Considering water pump failure reports here, it's prudent replace the engine water pump and associated thermostat at 100K miles.

The inverter water pump I think is similar construction? Likely best to replace that before long as well. To argue against replacement at the same interval: perhaps it's not used as much, or for some reason just lasts better. There don't seem to a lot of failures, at least not reported here. Also, it's a sandpounder to access.

BTW: with over 100k miles, I'd do a full EGR system cleaning ASAP, see top two links in my signature. (on a phone turn it landscape to see signatures)

 

FWIW: severe overheats can cause leaks in the seams of your plastic coolant overflow bottle. Your engines' normal operating temperature range is around 190F-215F and that overtemp. lamp goes off around 245F. You've overheated for a while to throw a P0117 - long enough for your ECU to think there was a temperature sensor issue.

 

This is one of those times you do it now. You are close to having to replace the engine. Change the water pump with oem now.