The Mercedes-Benz G550 tail light utilizes P21/5W and 12821 bulb sizes for the rear lighting components. Data for the specified vehicle covers the range from 2009 through 2018.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The P21/5W is the most frequently specified bulb type across the tail light data for the Mercedes-Benz G550, appearing in the outer tail light position across model years 2009-2018.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base. The dual-filament design allows a single bulb to serve two lighting functions, typically stop and tail.
The BAY15d bayonet base uses an offset pin configuration, which physically prevents incorrect installation of single-filament bulbs in sockets designed for dual-filament use.
The 12821 is a Philips part number designating a miniature wedge-style bulb with a W2.1x9.5d base, rated at 12V and 5W. This single-filament type serves a lower-wattage role compared to the P21/5W.
The W2.1x9.5d base of the 12821 is a wedge-style fitting, which is mechanically incompatible with the BAY15d bayonet socket used by the P21/5W. The two bulb types are not interchangeable.
For the Mercedes-Benz G550, the 2009-2010 model years carry a 12821 specification in one of the tail light sub-positions, while the 2011-2018 model years specify the P21/5W in that same position.
The outer tail light position on the G550 specifies the P21/5W uniformly across the 2009-2016 model years, indicating no change in socket type or wattage requirement over that span.
Because the 12821 and P21/5W differ in both base type and wattage output, substituting one for the other in a given socket would result in a physical fitment failure or incorrect light output.