Engine Heat Temp light comes on after radiator flush

[southpaw]

Hello Yaris world,

We just got used a 2007 Yaris with 88k miles and have an engine heat temp light that keeps coming on after driving a short distance or idling for a short period of time.

It ran fine until today when we decided to change the engine oil to full synthetic and flush the coolant in radiator. We changed the coolant because it didnt look like there was any in there, so we flushed it and added supertech 50 50 coolant and pre mixed with water. I know the manual recommends Toyota specific coolant but all the manufacturers do that and I never had any problems with vehicles in the past.

Do I need to flush the coolant and get toyota specific coolant? Could the thermostat be stuck or have gone bad during the coolant change? Any ideas what else it could be?

Thanks in advance for your help.

EDIT = Just pulled the thermostat out and tested it in boiling water and it opens up. Pull the stat out and run under cold water and it closes. Kind of points me towards toyota proprietary coolant being the issue. Any confirmation on this?