My 2008 Prius after two hours of Interstate (not city) Driving, has the following symptoms, the heater will stop working and the red triangle will come on. The car still drives but I have no heat. Different mechanics have drained the radiator/coolant at least three times and the Water pump has been changed. Furthermore, the heater bypass valve has been changed twice. I am at my wits end if anyone has any suggestions please let me know. Thank you