The Citroen DS21 front and rear turn signal bulbs both utilize a 1157 size bulb for proper operation. The replacement chart covers 1969 to 1972 production years with corresponding bulb size 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 DS21 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 specification used across many vehicle manufacturers.
The 1157 uses a BAY15d bayonet base, which is a dual-contact bayonet mount with two offset pins positioned at different heights to prevent incorrect insertion orientation.
As a dual-filament bulb, the 1157 houses two separate filaments within a single envelope, allowing it to serve two distinct lighting functions from one unit.
The 1157 operates on a 12-volt electrical system, consistent with the standard automotive voltage architecture of the era.
The two filaments in the 1157 are rated at 21 watts and 5 watts respectively, with the higher-wattage filament typically assigned to the active turn signal function and the lower-wattage filament to a secondary function such as parking or running light.
The shared 1157 specification across both front and rear positions means the same bulb type serves all four turn signal sockets on the DS21.
The BAY15d base is not interchangeable with single-contact bayonet bases such as the BA15s, as the dual-contact design carries separate circuits for each filament.