Why does the heater blow heat while driving and cold at idle?

The most likely cause of this issue is a problem with the car's coolant flow. When the car is driving, the water pump circulates the coolant throughout the engine and heater core, which allows the heater to blow hot air. However, when the car is idling, the water pump is not able to circulate the coolant as effectively, which can cause the heater to blow cold air.

There are a few things that could be causing this problem:

* Low coolant level: If the coolant level is too low, the water pump will not be able to circulate it properly.

* Faulty water pump: The water pump may be worn out or damaged, which can prevent it from circulating the coolant properly.

* Clogged heater core: The heater core may be clogged with debris, which can prevent it from allowing heat to transfer from the coolant to the air.

If you are experiencing this problem, it is important to have it diagnosed and repaired as soon as possible to prevent damage to your car's engine.