A car heater can fail for a number of reasons, but if it is blowing cold air, the two likely causes are that the coolant is not flowing through the heater core or that the air from the blower motor is not being routed through the heater core. Most of the time, you will have one of these two causes, although other underlying issues can cause a car heater to suddenly stop working.
Why is my heater blowing cold air, AND how do I fix the car heater that is blowing cold air?
Most cars on the road have water-cooled engines, and their heating systems work on the same basic principle. Hot coolant from the engine flows through a heater core, which looks and functions like a small radiator, and a blower motor forces air through it. The coolant then heats the air, which in turn heats the interior of the vehicle.
This is why it takes a while for the heaters to start blowing warm air. Until the engine is warmed up, there is no heat for the heater core to extract. It is also why a clogged heater core, a stuck thermostat, or air in the cooling system can cause a car heater to get cold.
The four main cooling system problems that can cause a heater to blow cold air are:
