The Mercedes-Benz CLK550 uses a 12961 bulb size for the front side marker and a P21/5W bulb size for the rear side marker. The replacement chart contains vehicle production years and bulb size specifications covering 2007 to 2009.
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 the 12961 bulb for the front side marker position across the 2007-2009 model years.
The 12961 is a Philips part number designating a T10 miniature wedge bulb with a W2.1x9.5d base, rated at 12V and 5W.
The T10 wedge base is a push-fit design, meaning the bulb is held in place by friction rather than a locking mechanism.
The rear side marker position on the CLK550 uses the P21/5W bulb across the 2007-2009 model years.
The P21/5W is an ECE-standard bulb, recognized across European and worldwide markets, with a BAY15d bayonet base and dual-filament construction rated at 12V, 21W and 5W.
The BAY15d base features two offset pins, which prevent incorrect insertion and ensure the correct filament is aligned with the intended circuit.
The front and rear side marker positions on the CLK550 use entirely different bulb types, with the front requiring a single-filament wedge bulb and the rear requiring a dual-filament bayonet bulb.
Because the 12961 and P21/5W differ in base type, wattage, and filament count, the two bulbs are not interchangeable between positions.
The dual-filament design of the P21/5W allows a single bulb to serve two separate lighting functions, with the 21W filament and the 5W filament operating independently.
The 5W rating of the front 12961 bulb reflects its role as a low-intensity marker light, consistent with regulatory requirements for side marker illumination.