The Audi S3 brake light uses 7440 and 921 bulb sizes for the rear lighting components. Data for the specific bulb sizes covers the range from 2015 to 2020.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 7440 bulb is the most frequently specified brake bulb size across the Audi S3 brake light fitments in these specifications, appearing in the brake position for model years 2015-2018 and 2020.
The 7440 is an ANSI-standard single-filament wedge bulb in the T20 form factor, rated at 12V and 21W, and is common in North American automotive applications.
The 2019 Audi S3 brake position deviates from the surrounding model years by using the 921 bulb rather than the 7440. The 921 is a T15 wedge bulb rated at 12V and 16W, drawing notably less wattage than the 7440.
Because the 921 and 7440 differ in both physical form factor and wattage, they are not interchangeable despite both being ANSI-standard wedge-type bulbs. Using the incorrect type in the 2019 Audi S3 brake position risks improper fitment or electrical incompatibility.
The outer brake position on the 2023-2024 Audi S3 uses the P21W bulb, which follows the ECE standard rather than the ANSI standard used in earlier model years.
The P21W uses a BA15s bayonet base, which is a physically distinct mounting style compared to the wedge base of the 7440 and 921. These bulb types are not interchangeable.
Although the P21W and 7440 share the same 12V, 21W electrical rating, their differing base types mean one cannot be substituted for the other without a socket modification.
The outer brake position for the 2019 Audi S3 uses the 7506 bulb, which is a separate specification from the brake position that year, indicating the S3 uses different bulb types across its brake light positions within the same model year.