Audi A3 Sportback e-tron front turn signal bulb size is P21W with 12174NA designation, while rear turn signal bulb size is 921 with 5008 designation. The replacement chart covers 2016 to 2018 production years with bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The P21W is an ECE-standard single-filament signalling bulb rated at 12V and 21W, fitted with a BA15s bayonet base. It serves as the front turn signal bulb on the Audi A3 Sportback e-tron for 2017-2018, specifically on trims equipped with HID headlamps.
The BA15s bayonet base is a single-contact base type, meaning the bulb makes electrical contact at one point at the bottom of the base. Both the P21W and the 5008 share this base design, though they differ in wattage and applicable standards.
The 12174NA is an OEM part number designation rather than an industry-standard code. It is a natural amber glass variant rated at 12V and is the specified front turn signal bulb for the 2016 model year.
Natural amber glass in the 12174NA produces a colored output directly from the glass envelope, rather than relying on an amber-tinted lens housing to achieve the required turn signal color.
The 921 follows the ANSI standard used in the US and Canada. It is a T15 wedge-base bulb rated at 12V and 16W, and it is the specified rear turn signal bulb for 2017-2018.
A wedge base, as used in the 921, is retained by friction rather than a twist-lock or bayonet mechanism, making it a distinct fitment type from the BA15s base found in the P21W and 5008.
The 5008 is an ANSI-standard single-filament bulb with a BA15s bayonet base, rated at 12V. It is described as a lower-wattage variant of the 1156 and is the rear turn signal bulb for the 2016 model year on trims with halogen capsule headlamps.
The front turn signal for 2016 uses the 12174NA, while 2017-2018 use the P21W. This represents a bulb type change across model years, and the two bulbs are not interchangeable due to differences in base type and glass specification.
The rear turn signal for 2016 uses the 5008 with a BA15s bayonet base, while 2017-2018 use the 921 with a wedge base. These two base types are physically incompatible, and substitution between them is not possible without a socket adapter.
The headlamp configuration on the 2016 model is relevant to bulb fitment. The front turn signal specifies HID headlamps for 2017-2018, while the 2016 rear turn signal specifies halogen capsule headlamps, indicating that headlamp type is a determining factor in the correct bulb selection for this vehicle.