Audi A4 Tail Light Bulb Size

The Audi A4 tail light bulb sizes include 921, P21W, 7506, and 1157 configurations for inner, outer, and standard tail light applications. A replacement chart documents bulb size specifications across 1996 to 2020 vehicle production years.

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

Inner Tail Light

Years	Bulb Size
2020	921
2019	921 (Without special body measures, Taillight assemblies, standard design)

Outer Tail Light

Years	Bulb Size
2020	P21W
2019	P21W (Without special body measures, Taillight assemblies, standard design)

Tail Light

Years	Bulb Size
2017-2018	921
2015	921 (For Vehicle With LED Taillights)
2009-2014	7506 (With Halogen Capsule Headlamps)
2007-2008	7506
1999-2002	1157
1996-1997	1157

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. 12V, 21W/5W.	1157

Other Locations

Additional Notes

The Audi A4 outer tail light draws from three distinct bulb types across its model history: the 921, P21W, and 1157, each belonging to different standardization systems and carrying different electrical characteristics.
The 921 is an ANSI-standard wedge bulb (T15) rated at 12V and 16W, used in the Audi A4 inner tail light for 2019-2020 and in the outer tail light for 2017-2018, as well as the 2015 model year on vehicles with LED taillights.
The P21W follows the ECE standard used in Europe and worldwide, features a BA15s single-contact bayonet base, and is rated at 12V and 21W. It appears in the outer tail light position for 2019-2020.
The 1157 is an ANSI-standard dual-filament bulb with a BAY15d bayonet base, rated at 12V with a 21W main filament and a 5W secondary filament. It is specified for the outer tail light on the Audi A4 for 1996-1997 and 1999-2002.
The dual-filament design of the 1157 means it serves two brightness levels within a single housing, a characteristic that distinguishes it from the single-filament P21W despite both sharing a 21W rating on their primary circuit.
The 921 and P21W are not interchangeable. The 921 uses a wedge base while the P21W uses a BA15s bayonet base, making them physically incompatible with each other's sockets.
For 2009-2014, the outer tail light on the Audi A4 specifies the 7506 bulb under the condition of halogen capsule headlamps, indicating that headlamp type is a determining factor in tail light bulb fitment for those years.
The 2007-2008 outer tail light also uses the 7506, though without any stated headlamp condition attached to those years.
The 2019 outer and inner tail light specifications both carry the qualifier "Without special body measures, Taillight assemblies, standard design," indicating that non-standard body configurations on that model year may require different bulb fitment.
The 2015 outer tail light specification applies specifically to vehicles with LED taillights, which typically use conventional bulbs only in certain positions within an otherwise LED assembly.