The Audi 4000 brake light uses a P21W bulb size for the rear lighting system. Data for the specific component covers the period from 1980 through 1987.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Audi 4000 uses the P21W bulb for its brake lights across the 1980-1987 model years.
The P21W is an ECE-standard single-filament bulb rated at 12V and 21W, fitted with a BA15s bayonet base, a single-contact base type widely used in European and international automotive lighting applications.
The BA15s base designation indicates a single-contact bayonet fitting with a 15mm collar, which is not interchangeable with the BA15d double-contact variant used in dual-filament applications.
The Center High Mount Stop Light on the Audi 4000 uses the 12961 bulb for the 1985-1987 model years.
The 12961 is a Philips part number referencing a miniature wedge bulb in the T10 form factor, rated at 12V and 5W, with a W2.1x9.5d wedge base.
The W2.1x9.5d base is a wedge-type fitting held in place by spring tension rather than a locking mechanism, making it physically incompatible with the BA15s bayonet base used by the P21W.
The wattage difference between the two bulb types is significant: the P21W operates at 21W for primary brake light duties, while the 12961 operates at 5W in the high mount stop light position, reflecting the lower output requirements of that secondary lamp.