5Q0 121 407M – Coolant Expansion Tank (Genuine)

Description

The 5Q0 121 407M Coolant Tank is a Genuine Volkswagen Audi OEM coolant reservoir engineered for MQB-platform Volkswagen and Audi models. Specifically, this tank performs three critical functions within the engine cooling circuit — it accommodates thermal expansion and contraction of coolant, maintains proper system pressure, and provides the primary fill point for the entire cooling circuit. Additionally, it houses the coolant level sensor, which triggers low-coolant warnings whenever fluid drops below the safe threshold. Because the factory designed this part to exact OEM specifications, it delivers a precise, leak-free fit using factory hose connections and mounting points with no modifications required.

Signs Your 5Q0 121 407M Coolant Tank Needs Replacing

Expansion tank failures follow a predictable pattern, so recognizing them early saves the engine from serious 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 pink residue beneath the front of the engine bay confirm active seepage. A persistent sweet coolant odor after driving is another strong indicator. Additionally, visible cracks or discoloration on the tank body confirm that replacement is overdue. In more advanced cases, air enters the cooling circuit through the compromised tank, and as a result the engine runs hotter than normal.

5Q0 121 407M Coolant Tank — Key Features

• OEM Reference: 5Q0 121 407M (superseded by 5Q0 121 407T / 5Q0 121 407AA)

• Brand: Genuine Volkswagen Audi — factory OEM part

• Includes cap and integrated coolant level sensor

• Maintains proper cooling system pressure and accommodates thermal expansion

• Precision-molded construction ensures a factory fit at all hose and sensor connections

Coolant Tank Service Notes

Before installation, confirm the new tank matches all hose port positions and sensor locations on the original unit. Next, fill the tank to the MAX mark using approved VW/Audi G12++, G13, or G13+ coolant only — never mix coolant types. After filling, run the engine to operating temperature and then recheck the level once it cools. Moreover, inspect all coolant hoses and clamps during this service and replace any that show cracking or softness. Finally, pressure-test the cooling system after installation to confirm a fully leak-free repair before returning the vehicle to service.

5Q0 121 407M Coolant Tank — Compatibility & Applications

Always verify fitment by VIN before ordering.

Cross Reference Numbers

• 5Q0 121 407A

• 5Q0 121 407D

• 5Q0 121 407F

• 5Q0 121 407G

• 5Q0 121 407S

• 5Q0 121 407T

• 5Q0 121 407AA

• 5QM 121 407B