Audi S8 inner tail light bulbs measure 12821 in size, while outer tail light bulbs measure P21/5W in size. The replacement chart contains vehicle production years and bulb size specifications spanning 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 tail light specifications cover the 2001-2003 model years, with two distinct bulb positions: the inner tail light and the outer tail light, each requiring a different bulb type.
The inner tail light across all three model years uses the 12821, a Philips part number designating a miniature wedge-style T10 bulb with a W2.1x9.5d base, rated at 12V and 5W.
The W2.1x9.5d base is a wedge-type connection where the glass envelope itself acts as the retaining mechanism, requiring no separate metal base to lock into the socket.
The outer tail light across the 2001-2003 Audi S8 uses the P21/5W, an ECE-standard dual-filament bulb with a BAY15d bayonet base, rated at 12V with filaments drawing 21W and 5W respectively.
The BAY15d base features two pins offset at different heights, a design that prevents incorrect insertion and ensures the higher-wattage filament is always oriented to the correct circuit.
The dual-filament construction of the P21/5W allows a single bulb to serve two functions simultaneously, typically combining a steady running light on the lower-wattage filament with a brighter stop or turn signal on the 21W filament.
The 12821 and P21/5W use entirely different base types, W2.1x9.5d and BAY15d respectively, making them physically non-interchangeable between the inner and outer tail light sockets of the Audi S8.