The Mercedes-Benz C350 tail light requires bulb size P21W or P21/5W depending on the specific version of the lighting component. Data for the bulb specifications covers the range from 2006 through 2010.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The P21W is the most frequently specified bulb across the Mercedes-Benz C350 tail light data, appearing in the outer tail light position across the 2008-2015 model years. It is a single-filament ECE-standard signalling bulb rated at 12V and 21W, fitted with a BA15s bayonet base.
The P21/5W is a dual-filament ECE-standard bulb rated at 12V with a 21W primary filament and a 5W secondary filament. Its base type is BAY15d, which is a bayonet fitting with two offset locating pins to prevent incorrect installation.
The P21/4W is a dual-filament ECE-standard bulb rated at 12V with a 21W primary filament and a 4W secondary filament. It uses a BAZ15d bayonet base, which has a different pin offset geometry than the BAY15d used by the P21/5W.
Because the P21W, P21/5W, and P21/4W each use a distinct base type, BA15s, BAY15d, and BAZ15d respectively, none of the three bulbs are physically interchangeable with one another despite sharing the same 21W primary wattage rating.
For the Mercedes-Benz C350, the outer tail light specification changed from dual-filament to single-filament bulbs between the 2007 and 2008 model years, when the P21/5W was replaced by the P21W.
The 2006 C350 outer tail light is specified with the P21/4W, which is the only model year in the dataset to use that bulb type in that position. Its BAZ15d base is unique among the three bulb types listed and is not compatible with sockets designed for the BA15s or BAY15d.
The 2007 C350 outer tail light is specified with the P21/5W, which uses a BAY15d base. This base features two pins at different heights, a design that physically prevents a single-filament BA15s bulb such as the P21W from being seated in its socket.
All three bulb types in these specifications conform to ECE regulations, a lighting standard used in Europe and numerous other markets worldwide. This classification indicates the bulbs meet defined photometric and safety requirements for road use in those regions.