The Mercedes-Benz CL500 brake light requires a P21/4W bulb size for proper installation and function. Data for the specified component covers the period from 1998 through 1999.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz CL500 uses the P21/4W bulb for its brake lights across the 1998-1999 model years. This bulb follows the ECE standard, which is the regulatory framework governing lighting equipment in Europe and most worldwide markets outside North America.
The P21/4W is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope. This construction allows one filament to handle a higher wattage function and the other a lower one, which is why the designation reflects both 21W and 4W ratings at 12 volts.
The P21/4W uses a BAZ15d bayonet base. The BAZ15d base features offset pins, a design that physically prevents the bulb from being inserted in the wrong orientation. This offset is critical because reversing the filament positions would assign the wrong wattage to each lighting function.
The center high mount stop light on the 1998-1999 CL500 uses the H21W bulb. Unlike the P21/4W, the H21W is a single-filament bulb, reflecting its dedicated use in a stop-light-only application rather than a combined function.
The H21W operates at 12 volts with a 21W output and uses a BAY9s base. The BAY9s is a bayonet-style base with asymmetrically placed pins, which, like the BAZ15d, serves as an anti-misfit feature to ensure correct installation.
Although both the P21/4W and H21W share a 21W filament rating, the two bulbs are not interchangeable. Their base types differ, with the P21/4W using BAZ15d and the H21W using BAY9s, making physical substitution between the two impossible without modification.