Subaru Ascent Back Up Light Bulb Size

The Subaru Ascent back up light requires bulb sizes 1157 and 921 depending on the specific version of the vehicle. The provided data covers the range from 2019 to 2024.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
2023-2024	1157
2022	921
2019-2020	921

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. 12V, 21W/5W.	1157
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921

Other Locations

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.