Audi S4 Brake Bulb Size

The Audi S4 uses multiple brake bulb sizes including 921, 7506, 1157, P21W, 2723, and 12961 across different brake light positions, with each bulb size corresponding to specific lighting functions on the vehicle. The replacement chart covers 33 years of S4 production data from 1992 to 2024, detailing bulb specifications for each model year.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Brake

Years	Bulb Size
2021	921
2020	7506
2018	7506
2015	921 (For Vehicle With LED Taillights)
2013-2014	7506 (For Vehicle With LED Taillights)
2007-2012	7506
2000	1157
1992-1994	1157

Center High Mount Stop Light

Years	Bulb Size
2007-2008	P21W
2000	2723
1992-1994	12961

Outer Brake

Years	Bulb Size
2023-2024	P21W
2019	P21W (Without special body measures, Taillight assemblies, standard design)

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. 12V, 21W/5W.	1157
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W
ANSI standard (US & Canada). Miniature wedge bulb (T5). 12V, 1.4W.	2723
Philips part number. Miniature wedge bulb (T10) with W2.1x9.5d base. 12V, 5W.	12961

Other Locations

Additional Notes

The 7506 bulb is the most frequently specified brake bulb across the Audi S4 data, covering model years 2007-2015 and 2018-2020. It is an ECE-standard single-filament bulb rated at 12V and 21W, functionally equivalent to the ANSI 1156 type, with a BA15s bayonet base.
The 1157 bulb, specified for the Audi S4 in model years 1992-1994 and 2000, is a dual-filament type with a BAY15d bayonet base rated at 12V and 21W/5W. Its dual-filament design allows it to serve both brake and tail light functions within a single bulb, unlike the single-filament types used in later years.
The P21W bulb shares the same BA15s bayonet base and 12V/21W rating as the 7506 but is classified under the ECE standard rather than the ANSI system. It appears in the Center High Mount Stop Light position for 2007-2008 and in the Outer Brake position for 2019 and 2023-2024.
The 921 bulb is a wedge-type (T15) ANSI-standard bulb rated at 12V and 16W. It is specified for the 2021 S4 brake light and also for the 2015 S4 specifically on vehicles equipped with LED taillights, where its lower wattage relative to the 7506 may reflect compatibility requirements of the LED taillight assembly.
The 2015 and 2013-2014 model years carry a conditional fitment note tied to LED taillight configurations. The 2015 S4 with LED taillights uses the 921, while the 2013-2014 S4 with LED taillights uses the 7506, indicating that the LED taillight variant does not uniformly dictate a single bulb type across those years.
The 2019 Outer Brake specification for the Audi S4 includes a fitment condition stating the P21W applies without special body measures and to standard-design taillight assemblies. This suggests alternative bulb requirements may exist for non-standard configurations of that model year.
The Center High Mount Stop Light on the 1992-1994 S4 uses the 12961, a Philips part number designating a T10 miniature wedge bulb with a W2.1x9.5d base rated at 12V and 5W. This is a significantly lower wattage than the brake bulbs used in the main stop light position on those same years.
The 2000 S4 Center High Mount Stop Light uses the 2723, a T5 miniature wedge bulb rated at only 12V and 1.4W. This is the lowest-wattage bulb across all positions in the specifications, reflecting the physically smaller and less prominent nature of some center high mount stop light housings of that era.
The 921 and 7506 are not interchangeable despite both being single-filament bulbs. The 921 uses a wedge (T15) base, while the 7506 uses a BA15s bayonet base, meaning the two require physically different sockets and cannot be substituted for one another without a socket adapter or housing modification.
The 1157 and P21W both use bayonet-style bases but are not interchangeable. The 1157 uses a BAY15d base with offset pins to accommodate its dual-filament design, while the P21W uses a BA15s base with a single filament. Inserting a single-filament BA15s bulb into a BAY15d socket is not possible without modification, and the wattage and filament configurations differ between the two types.