P1121 error code help!

Something else completely. It comes up fairly often here on PriusChat. You can do a search here to learn more about it.

That's the way warning codes are supposed to work. Best case, you get them earlier than any problem you would notice in driving the car.

 

On a Prius Hybrid this would indicate a stuck or slow moving coolant valve. The coolant valve moves and if the position sensor feedback on the valve doesn't indicate the valve is in the position it should be the vehicle will show a code that indicated the valve position isn't where it should be.

P1121 2007 TOYOTA PRIUS Coolant Flow Control Valve Position Sensor Circuit Stuck (With Video)

Did you change the coolant recently? It is not unusual for this valve to fail after a coolant change, it happened to me and I ended up buying a replacement valve and replacing this valve. If you are lucky the fault will reset and the valve won't stick again. You would need Techstream to reset fault.


another video : there are many on you tube if you search Toyota Prius 3 way cooling valve- a very common failure point

 

Or, it indicates that the valve isn't stuck or slow moving, but its position sensor does not report its position accurately.

As is always the case with a trouble code, one has to think about the detection condition—what the car's computer has to see to set this code—and the limits on what the computer is able to see. In this case, the computer is saying "hey, I told this valve to move to position X, but the signal coming back from the position sensor doesn't say it's at X." Naturally, the reason for that could be anything from the valve being mechanically stuck or slow, to the position sensor, to the wiring between the computer and the valve and sensor. (In super rare cases it could be a problem with the computer itself.)

Those are the possibilities it's always necessary for the human to go through, process of elimination, because the computer itself doesn't know which of those is the real cause.

As it happens, potentiometers used as position sensors in the Prius contribute to a number of familiar problems, so it's not unlikely to be at the bottom of a P1121.