Genuine VW/Audi 4K0121405N Expansion Tank/Coolant Reservoir – Audi | 4K0121405AA AU4K0121405N

The expansion tank is the reservoir that holds excess coolant until needed by the engine and allows for the expansion and contraction of coolant as it heats up and cools down.

Expansion tanks are often the highest point in the cooling system of your Audi, whether attached to the radiator or set apart in the engine compartment and connected to the cooling system using a hose. This placement allows any air trapped in the cooling system to be pushed into the expansion tank, where it can escape, and allows coolant to be added to the cooling system without allowing air to enter.

Detecting a Faulty Expansion Tank

Expansion tanks fail when they can no longer hold enough air pressure to prevent the cooling system from overflowing or when they can no longer hold coolant. Expansion tanks that are not holding pressure allow the cooling pressure to release, lowering the coolant’s boiling point and allowing the coolant to expand and aerate and the engine to overheat. Expansion tanks that are leaking coolant allow air to enter the cooling system instead of coolant, causing engine overheating. Steam coming from the expansion tank or coolant dripping from the expansion tank are signs that it has failed.

Failed expansion tank caps can mimic an expansion tank that does not hold pressure. Checking the expansion tank cap and replacing it may solve the problem of an expansion tank not maintaining pressure.

Replacing Your Audi Expansion Tank

Replacing an expansion tank is usually simple. With the exception of expansion tanks mounted to the radiator, crimping the hose from the expansion tank to the cooling system will allow you to unbolt, remove, and replace the expansion tank without draining and refilling the cooling system.

Audi Expansion Tanks.