The Geo Storm brake light requires a 1157 bulb size for proper installation and function within the rear lighting assembly. Data for the specific bulb requirements covers the period from 1990 through 1993.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Geo Storm uses the 1157 bulb for brake lights across all four model years of its production run, from 1990-1993.
The 1157 is a dual-filament bulb, meaning it serves two lighting functions within a single unit. In brake light applications, the higher 21W filament activates under braking, while the lower 5W filament typically handles the tail light function.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that prevents incorrect installation by ensuring the bulb can only be seated in one orientation.
The center high mount stop light on the Geo Storm uses the 921 bulb for all model years from 1990-1993, a different bulb type from the standard brake lights on the same vehicle.
The 921 is a single-filament wedge bulb in the T15 form factor, rated at 12V and 16W. Its wedge base is held in place by tension from the socket rather than a bayonet locking mechanism.
Because the 1157 uses a bayonet base and the 921 uses a wedge base, the two bulb types are not interchangeable with each other despite both operating at 12V.
Both the 1157 and 921 conform to ANSI standards applicable in the United States and Canada, indicating standardized electrical and physical specifications recognized across those markets.