Mercedes-Benz SLK350 Back Up Light Bulb Size

The Mercedes-Benz SLK350 back up light utilizes 921 and P21W bulb sizes for the rear illumination system. Data for the specific bulb requirements covers the range from 2005 to 2016.

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
2012-2016	921
2005-2011	P21W

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The Mercedes-Benz SLK350 back up light specifications cover model years 2005-2016 and reference two distinct bulb types: the 921 and the P21W.
The 921 bulb follows the ANSI standard used in the United States and Canada. It is a T15 wedge-base bulb rated at 12 volts and 16 watts.
The P21W bulb follows the ECE standard used in Europe and worldwide markets. It is a single-filament signalling bulb with a BA15s bayonet base, rated at 12 volts and 21 watts.
The wedge base of the 921 and the BA15s bayonet base of the P21W are physically incompatible mounting types, meaning the two bulbs are not interchangeable without a socket adapter.
Beyond the base type difference, the 921 and P21W also carry different wattage ratings, 16W versus 21W respectively, which reflects a difference in light output between the two standards.
The Mercedes-Benz SLK350 used the P21W for model years 2005-2011, indicating ECE-standard fitment across that production span.
Starting with the 2012 model year, the SLK350 specification shifts to the 921 bulb, reflecting a change to ANSI-standard fitment that continues through the 2016 model year.
Because the bulb type changes between the 2011 and 2012 model years, confirming the correct model year before selecting a replacement bulb is necessary to avoid a socket mismatch.