GMC C2500 Back Up Light Bulb Size

The GMC C2500 back up light utilizes 1156, 2456, 3156, and P21W bulb sizes depending on the specific version of the vehicle. Available data for the lighting components covers the range from 1979 to 2000.

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
2000	3156
1999	2456
1998	3156
1993-1997	1156
1992	2456
1991	3156
1990	2456
1988-1989	1156
1979-1986	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament wedge bulb (T25). 12V, 27W.	3156
ANSI standard (US & Canada). Festoon bulb, specialty application. 12V.	2456
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The GMC C2500 back up light bulb specifications cover model years 1979 through 2000, with three distinct bulb types appearing across that span: the 1156, 3156, and 2456.
The 1156 is the most frequently specified bulb in the GMC C2500 back up light application, appearing across 1979-1989, 1993-1997, making it the dominant fitment across the vehicle's production history.
The 1156 is a single-filament bulb with a BA15s bayonet base, rated at 12V and 21W, and conforms to ANSI standards for the US and Canadian markets.
The BA15s bayonet base on the 1156 is a single-contact base, meaning it is not interchangeable with dual-contact bayonet variants such as the BA15d, despite a similar physical profile.
The 3156 is a single-filament wedge bulb of the T25 type, rated at 12V and 27W, and is specified for the GMC C2500 in 1991, 1998, and 2000.
The 3156 operates at a higher wattage than the 1156 (27W versus 21W), and its wedge base design is physically incompatible with the BA15s bayonet socket used by the 1156, making the two non-interchangeable.
The 2456 is a festoon-type bulb rated at 12V, classified as a specialty application bulb under ANSI standards, and is specified for the C2500 in 1990, 1992, and 1999.
A festoon bulb is held in place by contact at each end of its cylindrical body rather than by a base socket, which means it requires a dedicated festoon-style holder and is not interchangeable with either the 1156 or 3156.
The back up light bulb specification for the GMC C2500 changes multiple times across consecutive model years, including between 1998 (3156), 1999 (2456), and 2000 (3156), indicating that socket type verification is necessary before replacement.
All three bulb types specified for the GMC C2500 back up lights conform to ANSI standards applicable in both the United States and Canada, and all operate on a 12V electrical system.