P0117 code w/ overheat light, no power loss or actual overheating, and oddly no cabin heat

2004 Prius. I intermittently get the “problem” warning followed by the red overheating light. When pulling codes, it’s P0117 (engine temperature sensor low voltage). However, when this occurs, I pull over and there is no evidence of the car overheating. I checked the coolant level in the radiator (not the overflow) and it was maybe one cup low. I have now driven a few hundred miles like this with no ill effects. No power loss, no hesitation. Only two odd additions to this puzzle. Firstly, I have zero cabin heat. The second one, is for a few seconds when I start the car and for a few seconds after shut off the car, it sounds as if one of the pumps in the driver’s side front engine area is working a bit too hard (louder than usual).

 

I was thinking of draining, refilling, and bleeding burping the coolant system tomorrow.... and see what happens. While under there, I’ll look for leaks first and see if somehow air is being sucked into the system somehow. Any usual culprits?

I’ve owned this car since 2005. In 2015, it hit 299,999 miles and that’s where the odo stopped. No idea how many miles it has now. Since then it’s seen daily driving, road trips, long road trips between VA and FL (at least 10) and four trips from VA to various CA and OR towns and back to VA.

 

Now I also have a pending P1121 code.... 3-way valve if I am correct?