So I was driving home yesterday from my grandparents house, about an hour drive, and in the last mile or so, my heater stopped blowing hot air and when I parked, there was steam spewing from the coolant tanks overflow tube.

Is this the sign of a bad Thermostat? I've already replaced the otterstat a few months ago, so I know the fans are running when they should, but because of where the otterstat is located in the system, if the thermostat is frozen closed, hot water would never pass by the otterstat to activate it, correct?

Thanks,
Brandon