BMW 328i front and rear side marker bulbs utilize 12961 and P21/5W sizes for the front position, with P21/4W and P21/5W options available for the rear location. The replacement chart covers 1996 through 2000 model 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 BMW 328i side marker specifications reference three distinct bulb types across the 1996-2000 model years: the 12961, the P21/5W, and the P21/4W.
The 12961 is a Philips part number designating a miniature wedge bulb in the T10 form factor. It uses a W2.1x9.5d base and operates at 12V with a 5W output, placing it in the low-wattage single-filament category.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V, with filaments drawing 21W and 5W respectively. It uses a BAY15d bayonet base, which features two offset pins to ensure correct polarity orientation during installation.
The P21/4W shares the same ECE standard and dual-filament construction as the P21/5W but differs in its secondary filament rating, which draws 4W rather than 5W. It also uses a distinct base type, the BAZ15d, whose pin offset differs from that of the BAY15d.
Because the BAY15d and BAZ15d bases are physically different, the P21/5W and P21/4W are not interchangeable despite their similar appearance and shared 21W primary filament rating.
The front side marker on the BMW 328i uses the 12961 for the 1999-2000 model years and the 1996 model year, while the 1997-1998 model years use the P21/5W instead.
The rear side marker follows a different pattern, with the P21/4W fitted to the 1999-2000 and 1996 model years, and the P21/5W fitted to the 1997-1998 model years.
The front and rear positions share the P21/5W bulb for the 1997-1998 model years, making those the only years where a single bulb type covers both side marker positions.
The 12961 is exclusive to the front side marker position in these specifications and does not appear in any rear side marker application across the listed model years.
The wattage difference between the P21/5W and P21/4W is confined to the secondary filament, and substituting one for the other would also require overcoming the physical incompatibility of their respective bayonet bases.