Pontiac GTO Back Up Light Bulb Size

The Pontiac GTO back up light utilizes 1156, 1157, and P21W bulb sizes depending on the specific application requirements for the vehicle lighting system. Available data for the bulb sizes covers the historical span from 1964 through 2006 for the various configurations listed.

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
2004-2006	1156
1973	1157
1972	1156
1971	1157
1964-1970	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. 12V, 21W/5W.	1157

Other Locations

Additional Notes

The 1156 bulb is the most frequently specified backup light bulb across the Pontiac GTO model years listed, covering 1964-1970 (excluding 1971 and 1973), as well as 2004-2006.
The 1156 is a single-filament bulb operating at 12 volts and 21 watts, fitted with a BA15s bayonet base, and recognized under ANSI standards in both the United States and Canada.
The 1157 bulb is specified for the 1971 and 1973 Pontiac GTO backup lights. It is a dual-filament bulb rated at 12 volts with two wattage outputs of 21W and 5W, and it uses a BAY15d bayonet base.
Although the 1156 and 1157 share the same 12-volt operating voltage and a 21-watt primary filament output, they are not interchangeable. The 1156 uses a BA15s single-contact base, while the 1157 uses a BAY15d dual-contact base with offset pins.
The offset pins on the BAY15d base of the 1157 are a physical keying feature that prevents incorrect installation in a socket designed for the single-contact BA15s base of the 1156, and vice versa.
The 1972 GTO returns to the 1156 single-filament bulb after the 1971 model year used the 1157, indicating that the two bulb types were used in alternating years during the early 1970s portion of the production run.
Both the 1156 and 1157 are ANSI-standardized bulb types, meaning their physical and electrical specifications are governed by a common North American standard, which allows for consistent cross-brand sourcing.