X AUTOHAUX Coolant Radiator Reservoir Tank for Audi A3 8VA/8VF — 5Q0121407A Direct-Fit with Cap & 2-Pin Sensor

This replacement coolant reservoir restores the coolant storage and overflow functions of the cooling system, keeping working temperatures stable and helping prevent heat-related drivability issues. Manufactured to match OE fitment and mounting points, the tank accepts the cap and the two-pin sensor included, making it a straightforward option when the original plastic reservoir has cracked, leaked, or become discolored.

Which vehicles will this tank fit?

The part is designed for a broad set of VW Group vehicles from mid-2010s platforms. Check the part numbers and model years against your VIN to confirm compatibility before ordering.

• Part numbers covered: 5Q0121407A, 5Q0121407D, 5Q0121407G.
• Compatible models include Audi A3 8VA/8VF (2012–2020), Audi Q2 GAB (2016–2020), Audi Q3 F3 (2018–2020), Audi TT MK3 FV (2014–2020).
• Also fits Volkswagen Golf MK7 (2012–2020), Volkswagen Arteon 3H7 (2017–2020), Volkswagen Passat 3G (2014–2020), and Seat Ateca KH (2016–2020).

How does this replacement reservoir perform compared with the original?

Built from reinforced plastic and treated to resist corrosion, the X AUTOHAUX tank aims to match the durability and sealing properties of the factory unit. It includes the filler cap and a two-pin sensor interface so you retain coolant-level monitoring and pressure control functions.

• Durable material resists oxidation and deformation over time.
• Seals and mounting points machined to match original geometry for a snug fit.
• Includes cap and 2-pin sensor connection to preserve instrumentation.

What’s involved in replacing the coolant reservoir?

Replacement is a moderate DIY task that requires basic hand tools and a clean drain pan. Always let the engine cool completely, then relieve any residual pressure before disconnecting hoses or removing mounting bolts to avoid spills.

• Turn off engine and allow to cool fully before starting work.
• Remove mounting fasteners, drain or catch remaining coolant, and detach hoses carefully.
• Install new tank, reconnect hoses and sensor, tighten clamps, and refill coolant to the specified level.

Will installing this tank fix overheating issues?

Replacing a cracked or leaking reservoir eliminates coolant loss from the expansion system, which can be a cause of overheating and erratic engine temperature readings. However, overheating can have other causes—thermostat, water pump, radiator, or head gasket—so inspect the entire cooling system if problems persist.

• Stops coolant leaks from a damaged overflow/reservoir tank.
• Maintains correct coolant volume and pressure behavior under normal operation.
• If overheating continues, further cooling-system diagnostics are recommended.

Who is this for: This X AUTOHAUX reservoir tank is intended for owners and technicians servicing mid-2010s Audi, Volkswagen and SEAT models who need an OE-form replacement with cap and sensor. It suits DIY mechanics replacing a failed plastic expansion tank, independent shops performing cooling-system repairs, or fleet managers refreshing aging vehicles to prevent coolant loss and maintain reliable engine temperature control.