My 2012 Prius sometimes shows a high (coolant) temperature warning light. Symptoms: 1) always after cold start 2) Cabin heating stays cold (really cold) 3) Radiator stays cold (fan runs (observed 1 time) 4) After stopping engine (car fully off and on again (reboot), cabin heating starts working again. 5) No coolant leakage 6) No OBD error codes It happened the first time at the end of my warranty period, dealer checked it, had the coolant system pressurised and couldn't find anything. This is not happening all the time, but time between events decreases. Where to start diagnosing or fixing before my engine crashes.
It could be that the engine water pump is going bad. To diagnose, use a scanner to monitor the engine coolant temperature.
You don't have your miles, but kinda sounds like you are might have a sticking thermostat, or failing coolant pump, or both. Do you have any scan tools to check if the coolant pump is functioning correctly? That would be the place to start.
Yes, no error codes could be found with ODB2 reader (carscanner). WIll hookup my Techstream to search deeper. Cooing temperatures are fine when everything works well. When I had issues (no cabin heating), the cooling water temp fluctuated beteen 40 adn 60 degC. After restart it was ~90 deg C.
Techstream: waterpump RPM was higher than the target RPM. (3600 RPM vs 3000 RPM) Already bought a new water pump and thermostat and replaced it yesterday. The rotor of the new pump (Aisin) is spinning much smoother than the old pump. Removed the rotor from the pump housing, the magnet part is still intact, not broken as shown in some of the videos. I don't think it is a bad thing to have the water pump replaced, but I am not sure whether I solved the root cause TBH.
It looks like my experience when I tested my (normal, AFAIK) water pump back in 2021 was that it also reported back a slightly higher RPM than what Techstream said the ECM was targeting (looks like I was seeing 3300 for a 3000-rpm target). I'm not sure that amount of discrepancy isn't just normal. I don't get the impression that the ECM bothers doing feedback control to get any exact actual RPM. Just sends a certain target-speed pulse train to the pump and uses the feedback rpm to set codes if it's wildly off.
I don't believe these cars store an over temperature event that triggers the lamp. With engine temperatures between 40C-60C, then jumping to 90C - after restart/reset clearly indicates that the coolant pump wasn't moving coolant as it should have. Just my two cents...
