Subaru Ascent Back Up Light Bulb Size

Additional Notes

• The Subaru Ascent uses two distinct back up light bulb types across its production years: the 1157 and the 921, both of which conform to ANSI standards recognized in the United States and Canada.
• The 1157 is a dual-filament bulb rated at 12V with a 21W/5W output, mounted on a BAY15d bayonet base. The dual-filament design means the bulb serves two separate electrical circuits at different wattages within a single housing.
• The 921 is a single-filament wedge-type bulb in a T15 form factor, rated at 12V and 16W. The wedge base relies on tension from the socket contacts rather than a locking bayonet mechanism to hold the bulb in place.
• The 1157 and 921 use fundamentally different base types, a bayonet mount versus a wedge mount, making them physically incompatible with each other's sockets. One cannot substitute for the other without a socket adapter or wiring modification.
• The Subaru Ascent (2023-2024) requires the 1157 bulb for the back up light position.
• The 2019-2020 and 2022 model years of the Ascent require the 921 bulb for the same position.
• The wattage difference between the two bulbs is notable: the 1157 draws up to 21W on its primary filament, while the 921 draws 16W. This difference reflects distinct electrical load expectations between the two socket types.
• Because the bulb type changed between the 2022 and 2023 model years, the socket configuration in the vehicle's tail lamp assembly also differs across those production periods.