THIS IS WHY YOUR PRIUS FAILS HEAD GASKETS,

This has been hotly debated around here as a factor or not. Some say there is little thermal expansion others say yes. My car routinely sees coolant temperature differences from 150f-200f.

You will see the deniers pop up below.

 

I'm not buying that explanation until someone explains to me why it only seems to happen to Gen 3 cars, while the other generations go through the same on/off ICE cycling.

 

It is excessive oil burning in the gen3 along with an egr before the cat to quickly carbonize the fumes. I don't think thermal cycling is the root cause but it is the difference between a Corolla with a 1.8 and a Prius. I see bad rings, identified by Toyota as an issue and an egr intake on the dirty side of the egr, identified by Toyota as an issue, as the root causes in that order.