The Volvo S40 back up light utilizes a P21W bulb size for the rear illumination system. Data for the specific component covers the range from 2000 to 2011.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Volvo S40 uses the P21W bulb size across all back up light positions for model years 2000-2011.
The P21W is an ECE-standard single-filament bulb rated at 12 volts and 21 watts, fitted with a BA15s bayonet base.
The BA15s bayonet base is a single-contact fitting, which is a key distinction from the BA15d double-contact base used in other automotive bulbs. Substituting a double-contact bulb into a single-contact socket is not compatible.
For the 2000-2004 Volvo S40, the P21W specification applies separately to both the inner and outer back up light positions, meaning each position draws on the same bulb type.
The 2005-2011 Volvo S40 does not carry a separate inner and outer sub-part distinction in the specifications, with a single back up light bulb position listed for those years.
The P21W designation follows ECE (Economic Commission for Europe) regulations, reflecting a standardized bulb type used across European and international vehicle markets rather than SAE-governed North American standards.
At 21 watts, the P21W produces a relatively high output for an incandescent signal bulb, which is consistent with its use in reverse lighting where adequate illumination behind the vehicle is required.