Newbie here: Just bought my first 2010 Prius.

Old coolant breaks down the head gasket.... Or so it's been told...

 

It’s a mystery wrapped in an enigma

 

Apparently everything breaks down the HG, including looking at it, thinking about it, contemplating it, talking about it, not talking about it, and the list goes on and on.

 

My money's on clogged EGR.

 

I'll take that money (and put it in my EGR cleaning fund)!

 

there's also a tsb for bad water pump speed software, allowing hot spots in the block and head

 

Love it when people think their Prius is the exception to the egr cleaning rule and prop theirs like a unicorn.