Mercedes-Benz SL550 Back Up Light Bulb Size

The Mercedes-Benz SL550 back up light requires a P21W bulb size for proper installation and function within the rear lighting assembly. The provided data covers the range from 2007 through 2012 for the specified vehicle components.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
2011-2012	P21W
2007-2009	P21W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

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Additional Notes

The P21W is the sole back up light bulb type listed for the Mercedes-Benz SL550 across all specified model years.
The P21W is an ECE-standard single-filament signalling bulb rated at 12 volts and 21 watts, designed primarily for signalling functions such as reverse lighting.
The P21W uses a BA15s bayonet base, which is a single-contact bayonet fitting with a 15mm collar. The "s" designation indicates a single contact pin, distinguishing it from dual-contact variants.
Because the BA15s base is single-contact, it is not interchangeable with the BA15d base, which carries two contact pins for dual-filament applications.
The ECE designation on the P21W indicates compliance with United Nations Economic Commission for Europe regulations, a standard recognized across European and many international markets.
The SL550 is specified with the P21W for model years 2007-2009 and 2011-2012, with each of those years sharing identical back up light bulb requirements.
The 21-watt rating of the P21W places it in a category commonly associated with high-visibility signalling applications, as opposed to lower-wattage bulbs used in parking or position lights.
The single-filament construction of the P21W means the bulb serves one lighting function only, consistent with its use in a dedicated back up light circuit rather than a combined stop and tail application.