when the car starts to over heat
FRICTION
the way the heat enter a car is sort of like a green house. when sunlight is traveling through glass some of the sun heat enter and some reflect back. the heats bulits up and heats up your car. If your car was black or a dark color it will obsorb the sun heat so if you were to touch the car it will most likely burn your finger. the heat will come out of your car if you were to open it.
you have a blown head gasket
It's a warning sign that you are low on coolant.
The heater in your car uses waste heat from the engine to heat the car. When the engine is not running there is no heat to distribute.
It shields the rest of the car from the heat of the exhaust or radiator .
If you have the AC turned on the fan will come on to help cool the engine from the load on the engine. If your car has dual fans only one should come on at that time and the other one would come on when the sensor obtains the required heat range.
A car engine is a heat engine, but there are various types of heat engines outside the realm of car and truck engines.
Car not blowing air or heat
The heat will also work when the car is turned on, but not moving. It uses heat from the engine.
You can use a paste flux and heat (as from a soldering gun) to reattach the solder.