Buy OEM 2003 Audi Part # 8J0121324 (8J0-121-324). Engine Coolant Reservoir Cap. 2.5 LITER, AUXILIARY. 4.2 LITER, RETURN. 4.2 LITER, RETURN, A4/S4. 6.0 LITER, RETURN, FROM VIN 4E6011643. 6.0

Engine Coolant Reservoir Cap. Plug.
A cover that prevents foreign debris from entering the container. Included with: Lower hose.

Engine Coolant Reservoir Cap

Fits A8, S4, TT, TT RS, TTS

2.5 LITER, AUXILIARY. 4.2 LITER, RETURN. 4.2 LITER, RETURN, A4/S4. 6.0 LITER, RETURN, FROM VIN 4E6011643. 6.0 LITER, RETURN, TO VIN 4E6011642.
Engine Coolant Expansion Tank - Repair or Replace
A failed expansion tank can cause the cooling system in your 2003 Audi to leak antifreeze, or it can cause your engine to overheat. If you're losing coolant, contact our service department to avoid engine damage.