My 2 year old, well maintained, regularly serviced Ford Focus sedan displayed an 'engine overheating' warning light and the fan ran continuously for at least a minute after the car was turned off. I returned it to the dealership I bought it from, they told me there was absolutely nothing wrong with it, I requested a further investigation at which point they said the warning light was the result of recent rain disrupting sensors. The problem occurred again, the coolant reservoir was empty and when the service dept checked the car again and found that the heater core needs to be replaced as there was a water leak. Why would a car in good condition need a heater core replaced? What would have caused the water leak?