Pressure relief valve in cap, checking

WARNING: This page is about a different car, the 2008 Audi RS 4 and 2007 Audi RS 4. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Checking Pressure Relief Valve In Filler Cap

Courtesy of VOLKSWAGEN UNITED STATES, INC.

Position cooling system tester V.A.G 1274 with adapter V.A.G 1274/9 on cap.
Generate a positive pressure using hand pump of cooling system tester.

Pressure release valve must open at a positive pressure of 1.4 to 1.6 bar.

If check-valve does not open as indicated:

Replace cap.