Water coolant valve? P1121 can I DIY?

No.

 

That's really hard to answer. Just about anyone can turn a wrench, and the job itself isn't that hard. You don't need to know anything technical about cars as long as you can follow directions to be honest. In addition to some common tools such as a socket wrench, an extension or two, and metric sockets, you will definitely want a set of cable operated hose clamp pliers. You will also be changing out the coolant, so you'll need that. I truly believe anyone can work on their own car if they just have good instructions, the correct tools, and a little common sense and a willingness to "just go for it." Who knows. It may be something you discover you love to do (like me). It is the best feeling when I finish a job and say "I just saved myself $500." In fairness I know plenty of people who would MUCH rather spend the $500 and they say "I just saved myself a bunch of work." So pick your side and go for it.
Give me a little bit and I'll find the post I followed to do mine.

 

P1121 - Coolant Control Valve Replacement (with pics) | PriusChat is what I used.