Prius Gen 3 temperature light issue

how is the oi level?
you may need a prius aware scanner to see the code
maybe a loose connection or chewed wire somewhere.
maybe the water pump is starting to fail
might be wise to monitor the engine temp

 

First off; the over temp lamp doesn't set a code and triggers around 240F.
Potential issues, electric water pump issue, radiator cooling fans not turning-on, obstruction or clogged (junk in your cooling system), and low coolant. Did you change your coolant per OEM service manual intervals?
1. electric water pump issue: not spinning fast enough to keep engine cool (need OBD2 scanner). They're know to give out around 150K miles. Need a qualified professional to check this.
2. radiator cooling fans not turning-on: you can turn-on AC and set cabin temp to Lo - both fans should turn on.
3.obstruction or clogged: this happens when coolant isn't changed or someone mixes non-compatible coolant into your system. Need a qualified mechanic to check this.
4. low coolant: you stated that you already checked this.

I would get a Bluetooth OBD2 dongle and upload a free Prius compatible software on your phone. You can then monitor engine operating temperatures. Normal operating temps are 190F-205F; if your constantly running your engine above those temps, you'll eventually pop the head gasket. You only have a 'dummy' lamp on that car, so you could be running the engine at 235F all the time without tripping the lamp.

Hope this helps......

 

For overheat probably not. With proper software the pump can be called up.manually. you can hear it chunking out etc . Without the software you can't do that

 

^ +1 for monitoring the engine coolant temperature. Linked below is a recent thread about using a display to monitor the coolant temperature.
https://priuschat.com/index.php?posts/3533114

 

Miles when was coollant drained and refilled?

 

How long have you had it? If it’s a recent acquisition seller may have added stop-leak product to engine coolant.

in the States, coolant change intervals are specified in Toyota Warranty and Maintenance Booklet. It can be downloaded from Toyota Tech info, in the manuals tab. I’ve attached a spreadsheet format summary, an easier read: both as published and extrapolated to 240k miles, Excel and PDF versions.

3rd gen, electrically driven water pump is proving failure prone, at 100~150k miles: safest is to replace both water pumps (engine and inverter) every 100k miles, plus the engine coolant thermostat.

the EGR system is prone to carbon clogging, which may lead to engine overheating, eventual head gasket failure, the nature of failure typically coolant leaking into combustion chambers. This is happening anytime after 100k miles (occasionally even sooner, typically in 150k~200k mile zone. I’d recommend to clean complete EGR system, including intake manifold, at least every 50k miles. More info in too two links in my signature. (On a phone turn it landscape to see signatures)

 

FYI, there are two separate coolant temperature sensors that can turn on the high coolant temperature warning light. The first is the sensor screwed into the engine. This is the sensor that most people think about. There is a second coolant sensor located in a tee fitting that is underneath the wiper crowl just behind the EGR valve. This sensor is for the coolant that flows through the exhaust heat recovery system. I will try to find and link a thread that explains this more.

Edit: Here is the link to the thread: https://priuschat.com/index.php?posts/3379685