The Mercedes-Benz CLK430 tail light utilizes the 12821 bulb size and the P21/4W bulb size for various lighting functions. Data for the specific components covers the period from 1999 through 2003.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The outer tail light of the Mercedes-Benz CLK430 uses three distinct bulb types across its 1999-2003 model years: the Philips 12961, the Philips 12821, and the ECE-standard P21/4W.
Both the 12961 and 12821 are Philips part numbers for miniature wedge bulbs of the T10 type, each rated at 12V and 5W and sharing the W2.1x9.5d base.
Despite carrying different Philips part numbers, the 12961 and 12821 share identical electrical ratings and the same base type, which may indicate a product line or series distinction rather than a difference in core electrical specifications.
The P21/4W is an ECE-standard dual-filament signalling bulb rated at 12V with a 21W main filament and a 4W secondary filament, fitted with a BAZ15d offset bayonet base.
The BAZ15d base of the P21/4W is an offset bayonet design, meaning the two pins are positioned at different heights to prevent incorrect installation of a single-filament bulb in a dual-filament socket.
The W2.1x9.5d wedge base used by both the 12961 and 12821 is a capless design where the glass envelope itself acts as the retaining element within the socket.
The P21/4W designation follows ECE (Economic Commission for Europe) naming conventions, indicating it is recognized as a standard automotive bulb type across European and many international markets.
The outer tail light position on the CLK430 uses both a wedge-type bulb and a dual-filament bayonet bulb simultaneously across the same model year in several cases, reflecting two separate bulb sockets within that single light assembly.
Because the P21/4W uses a BAZ15d bayonet base and the 12961/12821 use a wedge base, these bulb types are not interchangeable with one another regardless of voltage compatibility.
For the 2002 model year, the outer tail light of the CLK430 is specified with the 12961 in one socket position and the 12821 in another, meaning two physically similar but distinctly catalogued wedge bulbs occupy separate roles within the same assembly.