Mercedes-Benz C320 Back Up Light Bulb Size

Additional Notes

• The Mercedes-Benz C320 uses the P21W bulb size for the back up light across the 2001-2005 model years.
• The P21W is an ECE-standard bulb, meaning it conforms to European and international automotive lighting regulations rather than SAE standards used in North American specifications.
• P21W bulbs operate at 12 volts with a 21-watt output, which is a common rating for single-filament signalling applications such as reverse lights.
• The single-filament design of the P21W means it serves one lighting function only, distinguishing it from dual-filament bulbs that handle two functions such as braking and running light combinations.
• The P21W uses a BA15s bayonet base, where "BA15" refers to the 15mm base width and the lowercase "s" denotes a single-contact configuration.
• The BA15s base features offset locking pins that allow the bulb to seat and lock with a push-and-twist motion, a standard fitment method across many European vehicle platforms.
• Because the P21W follows ECE rather than SAE classifications, substituting it with a visually similar SAE-rated bulb without confirming equivalent wattage and base type risks an incorrect fitment.