What causes a car to overheat and blow cold air?

If the thermostat is plugged or stuck closed, the vehicle will usually run very hot or overheat quickly. But sometimes, if the thermostat is stuck open, the vehicle will not heat up to normal operating temp.(hard to tell in summer and not particularly harmful, unless using air conditioning, but very noticable in winter when the heater won't heat and the engine runs constant high idle).

A more visual way to check for certain is to remove the thermostat from the engine (usually a fairly simple procedure involving only partial draining of the radiator, a couple of bolts, a gasket, and some sealant). Put the thermostat in a pot of water on the stove and boil it. Using a thermometer, check the temp. at which the thermostat opens and closes (most vehicles 185 - 215 degrees farenheit). If no thermometer is available, then just the fact that the thermostat opens and closes around boiling tells you it's working (just not the specific operating temp. which matters in some vehicles and environments). However, as inexpensive (usually) as thermostats are, if one goes to the trouble of removing it from the car, one might as well buy a new one of the correct temp. range, install it in the vehicle and be done with it.