The Porsche 928 front and rear turn signal bulbs utilize either P21/5W or P21W bulb sizes depending on the specific model variant. The replacement chart covers 1978 through 1995 production years with corresponding 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 the dominant bulb type across the Porsche 928 turn signal specifications, appearing in both front and rear positions for the 1978-1994 model years, with the sole exception being 1995.
The P21W operates on a 12-volt circuit and draws 21 watts through a single filament, making it a dedicated signalling bulb with no secondary lighting function.
The P21W uses a BA15s bayonet base, which is a single-contact fitting. The "s" in BA15s denotes a single contact pin at the base, distinguishing it from dual-contact variants.
The 1995 Porsche 928 departs from the P21W specification, using the P21/5W in both the front and rear turn signal positions.
The P21/5W carries a dual filament, rated at 21 watts and 5 watts respectively, allowing it to serve two lighting functions from a single bulb housing.
The P21/5W uses a BAY15d bayonet base, which features two offset contact pins. This offset pin arrangement physically prevents the bulb from being seated in a BA15s socket, making the P21W and P21/5W non-interchangeable despite their similar appearance.
Both the P21W and P21/5W are governed by ECE regulations, a standard recognized across Europe and numerous other markets worldwide.
Because the front and rear turn signal positions share the same bulb specification in every model year listed, a single bulb type services all four turn signal housings on any given 928 from those years.