Heater Core
WARNING: This page does not describe the selected car, but rather 41 other vehicles, including the 2003 BMW M5, 2003 BMW M3, 2003 BMW 540i, 2003 BMW 530i, and 2003 BMW 525i. However, it is still accessible from the selected car via links, so may be relevant.
When the water valve is open, hot engine coolant circulates through the heater core. The heater core heats the air that passes through it; the hot air can then be used to warm the passenger compartment. The heater core (like the radiator) is also a "heat exchanger."
WARNING:
Danger of scalding! Work on the cooling system should only be carried out when the engine is cooled down.
CAUTION:
Open cooling system only when it is cooled down. Opening the cooling system while hot can result in air entering the system.
This can cause overheating with permanent damage to the engine
Recycling!
Catch and dispose of drained coolant. Observe country-specific waste-disposal regulations.