The heating system in your car works in one of two ways, both of which have a heater matrix at their core. In a water-valve heater, the temperature of the matrix is controlled by the amount of hot water that is sent through it; in an air-blending heater (more common in modern cars), the matrix remains at a constant temperature and warm and cool air are combined as mixing flaps open and close. Both systems are operated by the driver using a temperature control. Although the heater matrix in either system rarely malfunctions, it can become blocked or start to leak coolant. Leaking coolant has a sweet smell. If you notice a wet patch in the passenger-side foot well, check for the distinctive smell of coolant before you accuse your passenger of leaving the window open. The leak will be coming up from beneath the carpet. If your heater matrix is leaking or has become blocked, you will need to replace it.