Valves, Checking
WARNING: This page is about a different car, the 2022 Audi S5 Sportback, 2021 Audi S5 Sportback, and 2020 Audi S5 Sportback. However, it is still accessible from the selected car via links, so may be relevant.
-- Check the valves at stem and seat surface for wear grooves.
-- If there are clear wear grooves, replace the valve.