Prius intermittent temp light when driving uphill

Here's some screenshots of the two readings I got after climbing a hill.
Look at the Max temps on the upper left side of the gauges.

The exhaust heat coolant temp is higher after the ICE has been running, especially when the engine is in the Power Band of the meter on the dashboard.

 

It is major and you need to find out what it is and fix it.

 

Either air is in the system, and since you're hearing the coolant, it is very likely.
Or thermostat is sticking, or the coolant pump is failing, a hose is collapsing, or is clogged.
Or the radiator is clogged/blocked, or the coolant fans are failing or have failed....

Or any combination of the above.

 

People are sometimes kind of bad describing noises which sometimes are normal.

 

But in addition to the abnormal noises he is hearing, his car is overheating under load and/or high temperatures, which is also abnormal.

 

You have to measure both temperatures sensors at the same time. No codes and the first engine temp of 206f won't make the light come on.

You might be seeing a light other than the engine temperature such as the inverter temp which has a much lower trip point of 148f. This one will code and requires a reset to clear.

The two engine temps feed into the same light and won't code. It requires 248f at one of the two.

But I would guess you and Xliderider have stuck Exhaust Recirc valves which in your case is causing a quick engine temp flash. There is not enough heat added by the Recirc valve to raise the overall engine temp high. But it can and will flash the engine temp light from the second sensor as described on page 1.

Otherwise the prognosis is not so good including another head gasket leak. But to overheat with a hg means combustion gasses replacing the coolant which would be burning off. Coolant would be slowly going down; cold start rattles would occasionally be happening. Borescope inspection.