The BMW 323i tail light requires bulb size P21/4W or P21/5W depending on the specific version of the lighting component. Data for the bulb requirements covers the range from 1998 to 2000.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW 323i inner tail light uses two distinct dual-filament bulb types across its model years: the P21/4W and the P21/5W.
Both the P21/4W and P21/5W are ECE-standard bulbs, meaning they conform to European and worldwide automotive lighting regulations.
The P21/4W operates at 12V with a 21W/4W filament rating and uses a BAZ15d bayonet base, while the P21/5W operates at the same voltage but carries a 21W/5W filament rating with a BAY15d bayonet base.
Although the P21/4W and P21/5W appear similar in function, their bayonet bases differ: BAZ15d versus BAY15d. These two base types are not interchangeable, as the pin offsets are physically distinct to prevent incorrect installation.
For the BMW 323i, the 1998 inner tail light position lists the P21/5W, while the 1999-2000 model years list the P21/4W, indicating a base type change between those production periods.
The dual-filament construction of both bulb types means each serves two lighting functions within a single housing, typically combining a brighter stop-light filament with a lower-wattage running-light filament.