The 921 bulb is the most widely used type across the Audi SQ5 tail light specifications. It is an ANSI-standard wedge bulb classified as T15, rated at 12 volts and 16 watts, and is the standard for the North American market.
The P21W bulb is a single-filament signalling bulb with a BA15s bayonet base, rated at 12 volts and 21 watts. It follows the ECE standard used in European and worldwide markets, and its bayonet base makes it physically incompatible with the wedge socket used by the 921.
The lower tail light position on the Audi SQ5 uses the 921 bulb across the 2014-2017, 2019-2020, and 2023-2024 model years.
The upper tail light position uses the P21W bulb for the 2014-2017 model years, while the lower tail light position for those same years uses the 921. The two positions require different bulb types and are not interchangeable with each other.
The 921 and P21W bulbs differ in both base type and wattage. The 921 draws 16 watts through a wedge base, while the P21W draws 21 watts through a bayonet base. Substituting one for the other is not supported by these specifications.
The 2017 lower tail light entry carries a VIN restriction noting fitment up to VIN FP-H-150 000, indicating that the 921 bulb applies only to vehicles within that production range for that model year.
The 2019 and 2020 lower tail light entries include a "Bumper" designation, indicating that the 921 bulb in those years is specific to the bumper-mounted tail light position rather than the full tail light assembly.