High Temp Light coming on under heavy load

The thermostat right behind the electric water pump The water pump like if you can get the Toyota software to connect to your computer you can see if the water pump is turning decent RPMs when it's supposed to it will tell you that. The thermostat could be old and sticking intermittently. When you're running the car and you get it this hot to make the light come on if you whip over to the side of the road make a note of what hoses are hot or not at full temperature with the light on that sensor is thinking you're getting into the overheat range so I would expect hoses to be pressurized and pretty darn warm Make a note of which ones are not make a little diagram take a picture whatever it takes you may have serious vapor lock or water not moving because of air in it or something like that The Gen 3 is a head gasket candidate at some point and it's life usually earlier than later

 

thermostat?
maybe @ChapmanF has an idea at what temperature the warning light comes on?

 

See this thread. There are two engine coolant temperature sensors in Gen 3. The one in the EHRS hose will light the warning light at 248 ℉ (120 ℃). The one in the cylinder head can also light the light, and I haven't personally tested what the trigger temperature is for that one, but I'm assuming it's the same until somebody tells me it isn't.

 

I had this happen to me. In my case it was the water pump. While replacing, went ahead and also replaced the thermostat. Problem solved.

 

Have you replaced the brakes? Cause that might cause the ENGINE temperature light to come on.....

When you push the engine like you did, it WILL run hotter. But it really should not cause the light
to come on.
How many miles do you have on it?
Anytime the ENGINE temperature light comes on, it would be best to check things related to keeping
the temperature where it should be. Like the fans, water pump, coolant level, hoses, thermostat, etc....

 

Water pump is arguably a maintenance item on these cars so I wouldn't feel bad about throwing a new one in there at that mileage, even if it's not the real problem, which it sure seems like it is. If you go that route, make sure to use an AISIN pump from a reliable source and report back how your old one looks and spins compared to the new.

If you continue driving the car I suggest taking a proactive approach towards not getting the light to come on.

 

The brakes were a joke thing...
Since you said you just changed the coolant, are you 100% possitive you got all the air out?
Did you have this problem before changing it?

 

*** Please check the engine coolant pump motor, replace as a complete unit including the thermostat.

 

I live in the same place and im having the same overheating issue. I replaced the water pump, thermostat, and I cleaned the egr and air intake system. It still overheats when im going uphill. Did you ever fix your issue?

 

I pull a 4,500lb load at WOT achieving 18mpg at 65mph in Florida and don't overheat

 

I’m having this same problem: my light comes on when I’m driving over mountains (to/from Vegas, going north of LA) and going over 65 mph. I’ve noticed that if I keep the “hybrid system indicator” gauge out of the power mode, and preferably below the halfway line, the light won’t come on. So sometimes I have to slow down to like 30mph. Would love to know what’s causing this and get it fixed, but it doesn’t seem like any of these ideas have solved it for anyone yet?

 

How many miles on your Prius?

Your engine water pump is going bad!! Replace it ASAP. OEM water pump is preferred.