The Mercedes-Benz CLK350 inner tail light uses a P21/4W bulb size while the outer tail light requires a P21/5W bulb size. The replacement chart covers 2006 through 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 CLK350 uses two distinct bulb types across its tail light positions for the 2006-2009 model years: P21/4W in the inner tail light and P21/5W in the outer tail light.
Both the P21/4W and P21/5W are dual-filament bulbs operating at 12 volts, which allows a single bulb to serve two lighting functions simultaneously, such as running light and brake light duties.
The P21/4W carries wattage ratings of 21W and 4W across its two filaments, while the P21/5W carries ratings of 21W and 5W, making the secondary filament the only point of difference in power output between the two types.
Despite their near-identical wattage ratings, the P21/4W and P21/5W use different bayonet bases and are not interchangeable. 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 bulb installation in the socket.
Both bulb types conform to ECE standards, a regulatory framework recognized across Europe and numerous other markets worldwide, indicating standardized performance and safety requirements.
The P21/4W specification applies uniformly to the inner tail light position across all four model years covered, 2006-2009, with no variation in bulb size across that span.
The P21/5W specification applies uniformly to the outer tail light position across the same 2006-2009 range, also without variation.