The Smart Fortwo turn signal requires a PY21W bulb size for the lighting components located on the exterior of the vehicle. Data for the specific bulb requirements covers the range from 2017 to 2018.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Smart Fortwo uses two distinct bulb types across its front and rear turn signal positions: the PY21W and the 1157NA, each governed by different regional lighting standards.
The PY21W is an ECE-standard single-filament amber bulb rated at 12V and 21W, fitted with a BAU15s bayonet base. Its amber color is produced by the glass coating rather than a clear envelope.
The 1157NA is an ANSI-standard dual-filament bulb rated at 12V with a 21W high filament and a 5W low filament, fitted with a BAY15d bayonet base. Its natural amber glass provides the required color output.
The BAU15s base of the PY21W and the BAY15d base of the 1157NA are not interchangeable. The offset pin arrangement on the BAU15s differs from the straight dual-pin layout of the BAY15d, making cross-substitution physically incompatible.
For the Smart Fortwo front turn signal, the 1157NA was specified for model years 2008-2015, while the PY21W was specified for 2016-2018.
For the rear turn signal, the Smart Fortwo used the 7528 bulb for model years 2008-2015 and transitioned to the PY21W for 2016-2018.
The 7528 is a single-filament bulb sharing the same BAY15d base as the 1157NA, though it operates on a single filament rather than two. This means the 7528 and 1157NA are not drop-in replacements for each other despite sharing a base type.
The shift to PY21W across both front and rear positions for 2016-2018 represents a move to a single bulb type for all turn signal positions on the Smart Fortwo during those years.
The dual-filament nature of the 1157NA, used in the front turn signal position for 2008-2015, indicates that position served more than one lighting function during those model years.
Because the PY21W uses a BAU15s base with an offset pin, installation into a socket designed for a straight-pin BAY15d base is not possible without modification to the socket.