BMW 318i Back Up Light Bulb Size

Additional Notes

• The BMW 318i uses the P21W bulb size for the back up light across all listed model years from 1984 to 1998.
• The P21W is an ECE-standard bulb, meaning it conforms to the Economic Commission for Europe regulatory framework used in European and worldwide automotive lighting applications.
• P21W bulbs operate on a 12-volt electrical system and draw 21 watts, which is typical for single-filament signalling applications such as reverse lights.
• 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 point at the base, distinguishing it from dual-contact variants.
• Because the P21W is a single-filament bulb, it serves one lighting function only, which is consistent with its role as a dedicated back up light rather than a combined stop or tail light.
• The 1993-1998 model years of the BMW 318i share the same P21W specification, indicating consistent back up light fitment across that production period.
• The BA15s bayonet base is not interchangeable with the BA15d base, which carries two contact points. Substituting a dual-contact bulb in a single-contact socket would result in improper fitment or electrical incompatibility.