Audi 90 Back Up Light Bulb Size

The Audi 90 back up light utilizes a P21W bulb size for illumination. Data for the specific component covers the period from 1988 through 1995.

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

Bulb Size by Year Range

Years	Bulb Size
1993-1995	P21W
1988-1991	P21W

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

The P21W is the sole backup light bulb type specified for the Audi 90 across all listed model years.
The P21W operates at 12 volts and consumes 21 watts, placing it in the standard single-filament category for automotive signalling applications.
As a single-filament bulb, the P21W serves one dedicated function per housing, which in this application is the reverse light circuit.
The P21W uses a BA15s bayonet base, a single-contact fitting where the "s" denotes a single pin offset to one side, preventing incorrect rotational seating during installation.
The P21W is governed by ECE standards, a regulatory framework recognized across Europe and numerous other markets worldwide.
The Audi 90 (1988-1995) consistently uses the P21W specification across all listed model years for the backup light position.
The BA15s base is not interchangeable with the BA15d base, which carries two contact pins. Substituting a BA15d bulb in a BA15s socket is not possible without modification.
A 21-watt rating at 12 volts corresponds to a nominal current draw of approximately 1.75 amperes, a figure relevant to fuse and wiring circuit assessments.