Audi S3 front turn signal bulbs use 12174NA size while rear turn signal bulbs use PY21W or 921 size. The chart covers bulb size specifications across 2015 to 2024 production years for replacement purposes.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The front turn signal on the Audi S3 uses the 12174NA bulb across all listed model years, spanning 2015-2024. This bulb is identified by an OEM part number rather than a universal industry standard, and its natural amber glass produces the required turn signal color without relying on a colored lens or housing.
The 12174NA is rated at 12 volts, placing it within the standard low-voltage electrical architecture common to passenger vehicles. Its natural amber glass construction distinguishes it from clear-glass bulbs that depend on an amber-tinted socket or reflector housing to meet signaling color requirements.
The 2015-2016 front turn signal fitment of the 12174NA is noted specifically for vehicles equipped with HID headlamps, indicating that headlamp technology on the Audi S3 is a relevant factor in determining the correct front turn signal bulb for those years.
The rear turn signal on the Audi S3 uses two distinct bulb types across the listed years. The PY21W appears in 2015-2016 and again in 2023-2024, while the 921 appears in 2017-2020. These two bulbs are not interchangeable due to differences in base type, wattage, and standardization origin.
The PY21W follows the ECE standard used in Europe and worldwide markets. It is a single-filament amber bulb rated at 12 volts and 21 watts, fitted with a BAU15s offset bayonet base. The offset pin arrangement of the BAU15s base physically prevents installation of non-amber bulbs that share the more common BA15s base, serving as a built-in fitment safeguard.
The 921 follows the ANSI standard applicable to the United States and Canada. It is a T15 wedge-base bulb rated at 12 volts and 16 watts, producing approximately 5 watts less than the PY21W. The wedge base and the BAU15s bayonet base are mechanically incompatible, making the 921 and PY21W non-interchangeable without a socket adapter.
The 2019 rear turn signal entry specifies the 921 bulb for taillight assemblies of standard design, indicating that the Audi S3 in that year may have been available with more than one taillight assembly configuration, with the 921 applying only to the standard variant.
Across the full specification set, three bulb types are listed for the Audi S3 turn signal positions. Each originates from a different standardization framework, and none of the three shares a base type or wattage rating with either of the others, meaning cross-position substitution is not supported by the specifications.