Audi 5000 Back Up Light Bulb Size

The Audi 5000 back up light uses a P21W bulb size for the rear illumination system. Data for the specified lighting component covers the range from 1978 to 1987.

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
1978-1987	P21W

Bulb Specifications

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

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Additional Notes

The Audi 5000 uses the P21W bulb for the back up light position across all model years from 1978 to 1987.
The P21W is an ECE-standardized bulb type, meaning it conforms to the Economic Commission for Europe's regulations governing automotive lighting components used in Europe and internationally.
The P21W operates at 12 volts and 21 watts, which is a wattage typical of single-filament signalling applications such as reverse and turn signal lights.
The P21W uses a BA15s bayonet base, where "BA15s" denotes a single-contact bayonet fitting with a 15mm base width. The single-contact design is consistent with its single-filament construction.
Because the P21W is a single-filament bulb, it serves one lighting function per socket, as opposed to dual-filament types that handle two functions such as braking and running light simultaneously.
The BA15s base is not interchangeable with the BA15d base, which is a dual-contact variant of the same bayonet fitting. Substituting a BA15d bulb in a BA15s socket is not compatible due to the differing contact configurations.