Mercedes-Benz SLK300 Back Up Light Bulb Size

The Mercedes-Benz SLK300 back up light requires bulb size 921 or P21W depending on the specific application. Data for the bulb specifications covers the range from 2009 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
2016	921
2009-2011	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 SLK300 backup light specifications reference two distinct bulb standards across the model's production years: the ANSI-standard 921 and the ECE-standard P21W.
The 921 is a T15 wedge-base bulb rated at 12V and 16W, governed by ANSI standards applicable in the United States and Canada.
The P21W is a single-filament signalling bulb rated at 12V and 21W, governed by ECE standards used in Europe and worldwide markets. It uses a BA15s bayonet base rather than a wedge-type fitting.
The difference in base type between the 921 (wedge) and the P21W (BA15s bayonet) means the two bulbs are not interchangeable, as each requires a physically different socket.
The P21W draws 21W compared to the 921's 16W, reflecting a higher power rating associated with ECE-specification signalling bulbs.
The 2016 Mercedes-Benz SLK300 specifies the 921 bulb for the backup light position, placing it under the ANSI standard used in North American market vehicles.
The 2009-2011 model years specify the P21W, indicating those vehicles were fitted with ECE-standard lighting hardware at the backup light position.
The BA15s designation on the P21W denotes a single-contact bayonet cap with a 15mm base width, a common format in European automotive lighting applications.