The Mercedes-Benz CLK500 tail light requires a specific P21/4W bulb size for proper installation and function. Data for the bulb specifications covers the range from 2003 to 2005.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz CLK500 outer tail light uses two distinct bulb types across its production years: the P21/5W for the 2006 model year and the P21/4W for the 2003-2005 model years.
Both the P21/5W and P21/4W are ECE-standard dual-filament signalling bulbs rated at 12 volts, meaning each bulb carries two separate filaments operating at different wattages within a single housing.
The P21/5W operates at 21W on its primary filament and 5W on its secondary filament, while the P21/4W operates at 21W on its primary filament and 4W on its secondary filament.
Despite their similar naming and shared ECE standard, the P21/5W and P21/4W use different bayonet bases and are not interchangeable. The P21/5W uses a BAY15d base, and the P21/4W uses a BAZ15d base.
The BAY15d and BAZ15d bases differ in the offset positioning of their locating pins, a design feature that physically prevents incorrect bulb installation in a given socket.
The 2006 CLK500 outer tail light socket is therefore mechanically incompatible with the P21/4W bulb used in the 2003-2005 model years, as the BAZ15d base will not seat correctly in a BAY15d housing.