Genuine OEM Audi Heating System Reservoir, part number 4K1819552B. This reservoir is a crucial component of the vehicle's heating system, designed to store and circulate heated coolant to the heater core. It ensures a consistent supply of coolant, which is vital for maintaining comfortable cabin temperatures. Using a genuine OEM reservoir guarantees proper fluid capacity, leak-free operation, and compatibility with the vehicle's specific heating and cooling system, preventing potential issues with heating performance.