Audi S4 Turn Signal Bulb Size

Additional Notes

• The PSY24W is the front turn signal bulb for the Audi S4 across the widest span of model years in these specifications, covering 2009-2016, 2019, and 2021-2024. It is a single-filament amber halogen bulb operating at 12V and 24W, built to the ECE standard, and fitted with a PG20-4 base.
• The 7506 bulb served as the front turn signal for the 2007-2008 S4, placing it between the earlier ANSI-standard fitments and the later ECE-standard PSY24W era. The 7506 and PSY24W are not interchangeable due to differing base types and wattage ratings.
• The front turn signal for the 2000 S4 used the 3757NA, a dual-filament wedge bulb in a T25 form factor with natural amber glass, conforming to the ANSI standard used in the US and Canada. Its dual-filament design contrasts with the single-filament PSY24W used in later years, making the two types incompatible.
• The 1992-1994 S4 used the 1157 for the front turn signal, an ANSI-standard dual-filament bulb with a BAY15d bayonet base rated at 12V and 21W/5W. The BAY15d base on the 1157 is physically similar to the BAY15d base on the P21/5W, though the two standards are not directly interchangeable.
• The rear turn signal for the 1992-1994 S4 used the P21/5W, a dual-filament ECE bulb with a BAY15d bayonet base rated at 12V and 21W/5W. All other rear turn signal positions in these specifications use single-filament bulbs, making the P21/5W the only dual-filament rear fitment listed.
• The PY21W rear turn signal bulb, used on the S4 from 2018 through 2024 (and 2019 specifically for standard taillight assemblies without special body measures), carries a BAU15s bayonet base. The offset pin on the BAU15s base prevents it from being seated in sockets designed for the straight-pin BA15s base used by the P21W, so the two types are not interchangeable despite sharing the same 12V, 21W rating.
• For the 2013-2015 S4, the rear turn signal specification lists the P21W with a notation restricting it to vehicles equipped with LED taillights. The P21W uses a BA15s single-contact bayonet base and operates at 12V and 21W under the ECE standard.
• The 2019 S4 rear turn signal entry includes a fitment condition specifying standard taillight assemblies without special body measures, indicating that assembly variation within a single model year can affect bulb compatibility on this vehicle.