Almost guaranteed that it is the engine water pump.
Fairly common failure on high mileage Prii.

The inverter coolant loop is a whole different thing.

You can test the fans by turning the AC ON. The fans should run.

Since the water pump is electric, the "thermostat" function is accomplished by leaving it OFF until it comes up to the desired temprature. I'm pretty sure that there is NOT a conventional thermostat.

 

Thanks, Easy. I'll be working on this today. Hope to firm up the diagnosis and get the part ordered. I appreciate your help!

 

Ok, so here are the codes it's throwing: P1121, P1122, and P1116. Looked those up online, seems probable that it's the coolant valve, needs to be replaced. Once I took a look at mine, (after now noticing a nice patch of coolant leak on my driveway that I didn't notice till now) looks like there's definitely something up with it (see pics attached), and I'm guessing this is probably the culprit for both the red triangle of death / coolant temp light AND the no/slow hot air in cabin?

Question: I noticed also some discussion that made me think maybe I should also check to see my radiator isn't clogged... but I'm not sure how I'd be able to do that until after I replace the coolant valve and refill the system (esp since i seem to have a significant leak as well as a stuck valve?), then do the rev/hose squeeze method. Any good way to check before then? Or fix valve and assume no clog? I'm just not real familiar with coolant systems.

 

What does the "coolant valve" do ?
Water pumps often leak when they fail.
Please let us know how this turns out.