The Mercedes-Benz CLK550 front fog light requires H11 or H7 bulb sizes, while the rear fog light uses a P21/4W bulb size. The replacement chart covers 2007 to 2009 model years with corresponding bulb size 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 front fog light does not use a single bulb type across all model years. The 2007 and 2009 model years use the H11 bulb, while the 2008 model year uses the H7 bulb instead.
Because the 2008 CLK550 front fog light takes an H7 bulb rather than the H11 used in 2007 and 2009, the two bulb types are not interchangeable despite sharing the same wattage rating of 55W at 12V.
The H11 and H7 bulbs differ in their base design. The H11 uses a PGJ19-2 base, while the H7 uses a PX26d base. This physical difference means a bulb of one type cannot be fitted into a socket designed for the other.
Both the H11 and H7 are single-filament halogen bulbs operating at 12V and 55W. Each produces light through a single tungsten filament enclosed in a halogen gas environment, which extends filament life and maintains luminous output over time.
Both the H11 and H7 bulbs carry ECE certification, meaning they meet the regulatory standards applied across Europe and many other markets worldwide. Both are also approved for use in the United States, and the H7 additionally holds approval in Japan.
The rear fog light specification for the CLK550 is consistent across the 2007-2009 model years, with all three using the P21/4W bulb.
The P21/4W is a dual-filament signalling bulb rated at 12V with two power levels: 21W on the primary filament and 4W on the secondary filament. This dual-filament construction allows the bulb to serve two distinct lighting functions within a single unit.
The P21/4W uses a BAZ15d bayonet base. The offset pin arrangement of the BAZ15d base is designed to prevent incorrect installation, ensuring the two filaments are oriented correctly within the lamp housing.
Like the H11 and H7, the P21/4W carries ECE certification and is rated for use at 12V, making it consistent with the standard automotive electrical systems found in the CLK550.