The Audi 100 brake light utilizes a P21/5W bulb size for the lighting system. Data for the specified component covers the period from 1989 through 1994.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Audi 100 brake light circuit uses the P21/5W bulb across the 1989-1994 model years.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base. The dual-filament design allows a single bulb to serve both brake and tail light functions within the same housing.
The BAY15d bayonet base on the P21/5W features offset pins, which physically prevent incorrect installation of a single-filament bulb in its place. A standard BA15s single-filament bulb will not seat correctly in a BAY15d socket.
The center high mount stop light on the Audi 100 uses the 2723 bulb across the same 1989-1994 model years.
The 2723 is an ANSI-standard miniature wedge bulb of the T5 form factor, rated at 12V and 1.4W. Its low wattage reflects its role as a supplemental brake signal rather than a primary lighting source.
The T5 wedge base of the 2723 relies on the tension of the socket contacts rather than a locking mechanism, making it non-interchangeable with bayonet-type bases such as the BAY15d used by the P21/5W.
The P21/5W and 2723 bulbs follow different international standards, ECE and ANSI respectively, and are not interchangeable with each other in any position on the Audi 100.