'06 Prius... how far back and for how long are OBD codes stored?

Not to my knowledge. There is a "historical" status that some codes can be displayed with, but it certainly doesn't accumulate every code the car has ever seen, and I don't know what its exact rules are.

 

Per the OBD2 standard, some codes dont self erase and some do. The ones that do some take 40 warm up cycles and some take 80. (Not sure what a warm up cycle consists of on a hybrid though.)

 

The p1116 and p1121 i would think relate to a failed coolant flow control value, not a sensor.

 

They relate to a sensor of the position of the valve.

So when the ECM sends a command to the valve to move, and the sensor reading doesn't change, the ECM knows something is up. It doesn't know if the problem is with the valve or the sensor or the wiring to the sensor though. Still work for the human to do figuring it out.

 

ah good point.