The Mercedes-Benz S420 brake light uses P21/5W and P21/4W bulb sizes for the rear lighting system. Data for the specified vehicle covers the range from 1994 to 1999.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz S420 uses two distinct bayonet base types across its brake bulb fitments, making base compatibility a critical factor when sourcing replacements.
The P21/5W is the brake bulb type used in the S420 for model years 1995-1999. It is a dual-filament ECE-standard bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base.
The 1994 S420 uses the P21/4W instead of the P21/5W. While both are dual-filament ECE-standard bulbs rated at 12V with a 21W main filament, the P21/4W carries a lower 4W secondary filament and uses a BAZ15d bayonet base rather than the BAY15d found on the P21/5W.
The BAY15d and BAZ15d bases are not interchangeable. The offset pin positions on each base type differ, meaning a P21/5W cannot be substituted for a P21/4W, or vice versa, without a fitment mismatch.
The Center High Mount Stop Light on the S420 uses the H21W bulb for model years 1996-1999. This is a single-filament ECE-standard bulb rated at 12V and 21W, fitted with a BAY9s base.
The H21W's BAY9s base is a separate standard from both the BAY15d and BAZ15d bases used in the main brake positions, and the three base types are mutually incompatible.
All three bulb types specified for the S420 conform to ECE regulations, a standard recognized across Europe and numerous other markets worldwide.
The shared 21W main filament wattage across the P21/5W, P21/4W, and H21W reflects a common output level for signalling and stop-light applications, though the differing base designs prevent cross-position substitution.