Gen 3 Temp Light Came On / Heater Blowing Cold Air

You should read the codes to see what the car has stored. It could be anything, but temperature warnings would point you to coolant or waterpump/thermostat

 

If it's a temperature light it could be anything that's related to the coolant loops. In the Gen2 cars, the inverter coolant pump goes out every 100k miles or so. So you may be having the same issue. You'll need to read the codes to really know the issue, it'll help pinpoint the problem.

 

Probably air in heater core

 

No check engine light was mentioned, he can still look for hidden pending codes I guess.

Check to see if you have the auto button on and turn it off also keep the circulation button on.

Do you hear any water boiling sounds in the cabin?

 

Losing coolant (if that's the case here) is not a good sign for Gen3 Prius cars. Pay close attention to that coolant level and maybe as a preventative measure, clean out your EGR circuit and Intake Manifold areas. These cars are notorious for higher mileage head gasket failures, and it all starts from losing coolant.

 

That's the code for the engine's electric water pump spinning too fast.

Wait, you're thinking, why would the water pump spinning too fast be a problem?

Because it probably means the impeller inside the pump has separated into a couple of pieces, and it is spinning too fast because it doesn't pump much water that way.

 

[Generic comment on the reliability of different places to "read" things goes here.]

I have a 2010, which was before they added the code, so it is not covered in my repair manual. It was apparently added in the 2011 edition.

The car's repair manual is one of the better places to "read" things. They usually turn out to be true if you read them there.

On the other hand, I should confess that I didn't look it up in a 2011 manual myself; I have just taken Elektroingenieur's word for it.

That wasn't my first or the last time taking Elektroingenieur's word for something. It's theoretically possible that something you've "read" somewhere else is more reliable. You just have to ask yourself: do you feel lucky?

 

At 140k miles, you're due for the water pump and coolant change. Do them and you should be back on the road without issues (coolant related)

 

There is, of course, one way that code (which is for water pump overrevving) could be connected to low coolant.

If your coolant gets so low that the water pump isn't in it, naturally the pump will overrev.

That would be low coolant all right. The pump is pretty low on the engine.