They say it’s a blown head gasket

Your car is losing a lot of anti-freeze; unless it's visibly leaking I think your mechanic is correct that it's a blown head gasket. There won't be a "cheap and easy" fix like replacing the thermostat. I'm only surprised that you didn't list engine shake and misfires as symptoms.

 

Prius owners thinking it might not be a head gasket

 

are you losing coolant?

 

First, welcome back. A lot of water under the bridge since your last post in 2012. Head gasket failures on gen3s have become all too common these days. Normally they start out as occasional severe cold start rattles that clear in a few seconds. Over a period of weeks or months the severe shaking and metal to metal sounds become more frequent and coolant loss becomes obvious. Did you experience the cold start rattling?

In the end, an engine which has blown a head gasket and has repeatedly overheated needs to be replaced rather than attempting just a head gasket. Some diy types might try a hg alone since their labor is free. Otherwise most shops might test with a borescope camera in the cylinders looking for a steam cleaned piston or signs of coolant.

Otherwise a used engine is the lowest cost short term solution. Short term meaning who knows how long it might last. You could get lucky.

Next and preferably would be another engine without signs of hg failure rebuilt with new pistons rings and machined head.

 

Depending where you're located and what you can or cannot do JDM spec engines going to cost you about $1,200 then you have to get it to you. I also bought a low mileage transmission to go with the same engine you may be buying so I had two pieces of low mileage equipment to get that put in the car is going to cost you about $1,000 to $1,500 depending upon what kind of mechanic you deal with I paid $800. So for me it was relatively inexpensive got my car back on the road that's driving great All these failures that I'm seeing like ours are between the 180 mark and 250 2:30 somewhere in there. So I'm banking I can take this thing that I have with $37,000 to $70,000 mi on it all the way to the 240 I had on it when the original head gasket blew so then the car will have about 480,000 on it we will see. I'm also going to take a Gen 2 to about 600,000 we will see