The Audi 4000 front turn signal requires a P21/5W bulb size while the rear turn signal uses a P21W bulb size. This replacement chart covers vehicle production spanning 1980 to 1987.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Audi 4000 uses two distinct bulb types across its front and rear turn signal positions: the P21/5W for the front and the P21W for the rear, both conforming to ECE standards recognized in Europe and worldwide.
Both bulb types operate on a 12-volt electrical system, which is standard for passenger vehicles of this era.
The P21/5W is a dual-filament bulb, meaning it carries two separate filaments within a single housing, rated at 21 watts and 5 watts respectively.
The P21W is a single-filament bulb rated at 21 watts, serving exclusively as a turn signal source in the rear position.
The P21/5W uses a BAY15d bayonet base, which features offset pins to prevent incorrect installation of the dual-filament bulb in the wrong orientation.
The P21W uses a BA15s bayonet base, a single-contact fitting that is not interchangeable with the BAY15d base used by the P21/5W despite their similar physical appearance.
The front turn signal position on the Audi 4000 calls for the P21/5W across all model years from 1980-1987, while the rear turn signal position calls for the P21W across the same span.
Because the BAY15d and BA15s bases differ in pin alignment and contact count, substituting a P21W in a P21/5W socket, or vice versa, would result in an improper fit or incorrect electrical function.