Audi Q3 Brake Bulb Size

Additional Notes

• The P21W is the brake bulb size specified for the Audi Q3 across all listed model years.
• P21W is an ECE-standardized bulb type, meaning it conforms to the Economic Commission for Europe regulations that govern lighting equipment used in vehicles sold across European and worldwide markets.
• The bulb operates on a 12-volt electrical system and draws 21 watts, which is a standard power rating for single-filament signalling applications such as brake lights.
• P21W uses a BA15s bayonet base, a single-contact fitting where the "s" denotes a single pin offset, distinguishing it from double-contact variants like the BA15d.
• Because the BA15s base is a single-contact design, it is not interchangeable with physically similar bases that carry two contacts, even when the outer housing appears to fit the socket.
• The single-filament construction of the P21W means the bulb serves one lighting function only, as opposed to dual-filament types such as the P21/5W, which handle two functions from a single unit.
• The 2021-2024 and 2017-2018 model year groups each share the same P21W specification, indicating consistent brake bulb fitment within those respective ranges.