Chevrolet Trailblazer Back Up Light Bulb Size

Chevrolet Trailblazer back up light bulbs utilize 921 and 3157 bulb sizes depending on model generation. The replacement chart covers vehicle production spanning 2002 to 2024 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.

Bulb Size by Year Range

Years	Bulb Size
2023-2024	921
2021	921
2002-2005	3157

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
ANSI standard (US & Canada). Dual-filament wedge bulb (T25). 12V, 27W/7W.	3157

Other Locations

Additional Notes

The Chevrolet Trailblazer backup light uses two distinct bulb types across its model years: the 921 and the 3157, and these two bulbs are not interchangeable with each other.
The 921 bulb is a single-filament T15 wedge type rated at 12V and 16W, conforming to the ANSI standard used in the United States and Canada.
The 3157 bulb is a dual-filament T25 wedge type rated at 12V with a 27W main filament and a 7W secondary filament, also conforming to the ANSI standard used in the United States and Canada.
The dual-filament design of the 3157 means it carries two separate circuits within a single bulb, which is a fundamentally different electrical configuration from the single-filament 921.
The T15 and T25 designations refer to the tubular glass envelope size of each bulb, with T25 being physically larger than T15, which contributes to their incompatibility as direct substitutes.
The Chevrolet Trailblazer (2021-2024) uses the 921 bulb for the backup light position.
The Chevrolet Trailblazer (2002-2005) uses the 3157 bulb for the backup light position.
The wattage difference between the two bulbs, 16W for the 921 versus 27W on the primary filament of the 3157, reflects a meaningful difference in power draw that affects socket and circuit compatibility.