The Mercedes-Benz CLK550 inner tail light uses a P21/4W bulb size while the outer tail light requires a P21/5W bulb size. The replacement chart covers 2007 to 2009 model years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz CLK550 uses two distinct bulb types across its tail light positions for the 2007-2009 model years: P21/4W for the inner tail light and P21/5W for the outer tail light.
Both the P21/4W and P21/5W are ECE-standard dual-filament signalling bulbs rated at 12 volts, a standard voltage shared across both positions.
The P21/4W operates at 21W on its primary filament and 4W on its secondary filament, while the P21/5W operates at 21W on its primary filament and 5W on its secondary filament.
Although the P21/4W and P21/5W share a similar designation and dual-filament design, they are not interchangeable. Each uses a different bayonet base: the P21/4W uses a BAZ15d base and the P21/5W uses a BAY15d base.
The BAZ15d and BAY15d bases differ in the offset positioning of their locating pins, a design feature that physically prevents incorrect installation of one bulb type in place of the other.
Both bulb types conform to ECE regulations, indicating suitability for use in European and worldwide markets where ECE lighting standards apply.