The Audi S8 brake light utilizes a P21/5W bulb size for the specific lighting component located at the rear of the vehicle. Data for the bulb specifications covers the range from 2001 to 2003.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Audi S8 brake light positions covered in these specifications are the standard brake lights and the center high mount stop light (CHMSL), with each position using a distinct bulb type across the 2001-2003 model years.
The P21/5W is the brake bulb used in the Audi S8 for the 2001-2003 model years. It follows the ECE (Economic Commission for Europe) standard, meaning it is a regulation-compliant type recognized across European and international markets.
The P21/5W is a dual-filament bulb, which allows it to serve two separate lighting functions from a single unit. In brake light applications, the lower 5W filament typically handles a running light function while the higher 21W filament activates under braking.
The P21/5W operates on a 12V electrical system and uses a BAY15d bayonet base. The BAY15d base features offset pins, a design that physically prevents incorrect insertion and ensures the two filaments align with their intended circuits.
The center high mount stop light on the 2001-2003 Audi S8 uses the 12961, a Philips part number designation for a miniature wedge-style bulb. This is a single-filament unit rated at 12V and 5W, consistent with a dedicated stop signal function requiring no dual-filament capability.
The 12961 belongs to the T10 wedge bulb family and uses a W2.1x9.5d base. The wedge base design relies on the bulb's glass envelope and wire leads to create the electrical connection, with no separate metal base component as found on bayonet types.
The P21/5W and the 12961 use fundamentally different base types, a BAY15d bayonet and a W2.1x9.5d wedge respectively, making them physically non-interchangeable with each other regardless of voltage compatibility.
The wattage difference between the two bulb types is notable. The P21/5W draws up to 21W on its brake filament, while the 12961 draws only 5W. This reflects the different optical and housing requirements between a primary brake light assembly and a supplemental high mount stop light.