Citroen D21 front and rear turn signal bulbs both utilize a 1157 bulb size. The replacement chart covers 1969 to 1972 production years with corresponding specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1157 is the sole bulb type specified for the Citroen D21 across all listed positions and model years, covering both the front and rear turn signals from 1969-1972.
The 1157 is an ANSI-standardized bulb, meaning its physical and electrical characteristics conform to a defined North American standard used across the United States and Canada.
The 1157 is a dual-filament bulb, which means it contains two separate filaments within a single envelope, allowing it to serve two distinct lighting functions from one unit.
The bulb uses a BAY15d bayonet base, a two-pin offset base design where the two pins are positioned at different heights to ensure the bulb can only be seated in one rotational orientation.
The 1157 operates at 12 volts, consistent with the standard electrical system voltage found in most passenger vehicles of that era.
Its two filaments are rated at 21 watts and 5 watts respectively, with the higher-wattage filament typically serving the turn signal function and the lower-wattage filament serving a secondary role such as a parking or position light.
Because the same 1157 bulb is specified for both the front and rear turn signal positions on the Citroen D21, the two socket locations share a physically and electrically identical fitment requirement.
The BAY15d base is not interchangeable with single-filament bayonet bases such as the BA15s, as the dual-pin offset configuration is specific to dual-filament applications.