The Mercedes-Benz S500 brake light utilizes P21/5W and P21/4W bulb sizes for the rear lighting system. Data for the specified components covers the period from 1994 through 1999.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz S500 brake light circuit relies on two ECE-standard bulb types across its model years: the dual-filament P21/5W and P21/4W, and the single-filament H21W.
The P21/5W is a dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base. This offset-pin base ensures the bulb seats in only one rotational orientation, preventing incorrect filament alignment within the housing.
The P21/4W shares the same dual-filament, 12V, 21W primary rating as the P21/5W but carries a lower 4W secondary filament and uses a BAZ15d bayonet base. The BAZ15d pin offset differs from the BAY15d, making the P21/4W and P21/5W physically non-interchangeable despite their similar appearance.
The Mercedes-Benz S500 (1994) uses the P21/4W for the brake light, while the 1996-1999 model years use the P21/5W. The base difference between these two bulbs means a P21/5W cannot be substituted into a 1994 fitment without a socket mismatch.
The H21W is a single-filament bulb rated at 12V and 21W, fitted with a BAY9s base. As a single-filament type, it serves a dedicated stop-light function rather than a combined stop-and-tail role.
The Center High Mount Stop Light on the S500 uses the H21W for model years 1996-1999. This position is a separate circuit from the main brake lights and takes a different bulb type entirely.
All three bulb types listed in these specifications conform to ECE regulations, a lighting standard recognized across Europe and numerous other markets worldwide.
The BAY9s base of the H21W is a single-contact bayonet fitting, distinct from both the BAY15d and BAZ15d bases used by the dual-filament brake bulbs. None of the three base types in these specifications are cross-compatible with one another.