The Audi S4 inner tail light uses a 921 bulb size, while outer tail light and general tail light applications accommodate 921, 7506, and 12821 bulb sizes. The replacement chart covers vehicle production years from 1992 to 2024 with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 921 bulb is an ANSI-standard wedge type in the T15 form factor, rated at 12V and 16W, and it is the most frequently specified bulb across the Audi S4 tail light fitment data provided.
The 7506 bulb shares the same T15 wedge form factor and 12V/21W rating as the 921 in many international markets, though the two carry different ANSI designations and are not always interchangeable without verification.
The inner tail light position on the Audi S4 uses the 921 bulb across the 2019-2020 and 2023-2024 model years, with the 2019 fitment applying specifically to vehicles without special body measures and with standard-design taillight assemblies.
The outer tail light position shows a bulb type change between the 2023 and 2024 model years, shifting from the 7506 to the 921, which represents a wattage difference that may affect direct substitution between those years.
The 2019 outer tail light specification for the Audi S4 is conditioned on the absence of special body measures and the presence of standard-design taillight assemblies, indicating that vehicles with non-standard configurations may require a different bulb.
The 12821 is a Philips part number rather than an ANSI designation, covering a miniature T10 wedge bulb with a W2.1x9.5d base rated at 12V and 5W, a significantly lower wattage than either the 921 or 7506.
The 1157 is a dual-filament bulb with a BAY15d bayonet base, rated at 12V with a 21W main filament and a 5W secondary filament, making it physically and electrically distinct from the single-filament wedge bulbs used in other S4 model years.
The 2013-2015 outer tail light entries carry a notation specifying LED taillights, meaning the 921 bulb in those years is associated with a hybrid lighting assembly rather than a fully conventional one.
The outer tail light position on the Audi S4 has used three distinct bulb types across its model history, specifically the 1157 bayonet, the 12821 miniature wedge, and the 7506 or 921 T15 wedge, reflecting changes across generations that make cross-year substitution unreliable without confirming the exact model year.
The BAY15d bayonet base of the 1157 bulb is physically incompatible with the wedge sockets used by the 921, 7506, and 12821 bulbs, meaning socket type is a hard constraint when sourcing replacements for the 1993 and 2000 model years.