1K0 121 407F – Expansion Coolant Tank (Hudson)

Description

The 1K0 121 407F Coolant Tank by Metrix is a direct-fit OEM-style replacement engineered to restore dependable cooling system performance across a wide range of MK5 and MK6 platform Volkswagen and Audi models. Specifically, this tank performs three essential functions within the cooling circuit — it accommodates thermal expansion and contraction of coolant, maintains correct system pressure, and serves as the primary fill point for the entire circuit. Because Metrix engineers this tank to OEM dimensions, it installs directly using factory hose connections and mounting points without any modifications required. Additionally, this unit includes the integrated coolant level sensor, ensuring the complete assembly is ready for direct installation.

Signs Your 1K0 121 407F Coolant Tank Needs Replacing

Expansion tank failures follow a predictable pattern, so recognizing them early prevents serious engine damage. First, a recurring low-coolant warning — even after refilling — signals a slow leak from the tank body or seam. Furthermore, coolant puddles or dried residue beneath the front of the engine bay confirm active seepage. A persistent sweet coolant odor after driving is another strong indicator. Moreover, original tanks commonly become brittle, develop stress cracks, or discolor significantly from years of heat cycling. Additionally, a failed level sensor may trigger a false low-coolant warning, even when the system is full. In more advanced cases, air enters the cooling circuit through the compromised tank, and consequently the engine runs hotter than normal.

1K0 121 407F Coolant Tank — Key Features

• OEM Reference: 1K0 121 407F

• Brand: Metrix — OEM-style replacement parts

• Includes integrated coolant level sensor

• Direct bolt-on fitment using factory hose connections and mounting points

• Correct tank geometry for proper cooling circuit integration

• Engineered to restore proper system pressure and coolant volume control

Coolant Tank Service Notes

Before installation, confirm that all hose port positions and sensor connections on the new tank match those on the original unit. Next, fill the tank to the MAX mark using approved VW/Audi G12, G12+, or G13 coolant only — never mix coolant types, as doing so degrades inhibitor chemistry and accelerates internal corrosion. After filling, run the engine to operating temperature and recheck the coolant level once it cools completely. Moreover, inspect all coolant hoses and clamps during this service and replace any that show cracking, softness, or swelling. Finally, verify that the level sensor connector is fully seated and clear any stored coolant-level fault codes before returning the vehicle to service.

1K0 121 407F Coolant Tank — Compatibility & Applications

Always verify fitment by VIN before ordering.

Cross Reference Numbers

• 1K0 121 407A

• 1K0 121 407C

• 1K0 121 407D

• 1K0 121 407E

• 1K0 121 407G

• 1K0 121 407H