GMC K2500 Back Up Light Bulb Size

The GMC K2500 back up light utilizes bulb sizes 1156, 2356, 2456, 3156, and P21W depending on the specific application. Available information covers the range from 1979 through 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	2456
1999	3156
1998	2456
1993-1997	1156
1992	2356
1991	2456
1990	3156
1988-1989	1156
1979-1986	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Festoon bulb, specialty application. 12V.	2456
ANSI standard (US & Canada). Single-filament wedge bulb (T25). 12V, 27W.	3156
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.	2356

Other Locations

Additional Notes

The GMC K2500 back up light specifications cover model years 1979 through 2000, with the 1156 bulb appearing most frequently across that range.
The 1156 is a single-filament bulb with a BA15s bayonet base, rated at 12V and 21W, and it serves as the back up light bulb for the K2500 in 1979-1989 and 1993-1997.
The BA15s bayonet base on the 1156 is a single-contact fitting, meaning it is not interchangeable with dual-contact bayonet bases without an adapter or socket modification.
The 3156 is a single-filament wedge bulb in a T25 form factor, rated at 12V and 27W, and it is specified for the GMC K2500 in the 1990 and 1999 model years.
Because the 3156 uses a wedge base rather than a bayonet base, it requires a different socket than the 1156 and the two bulb types are not directly interchangeable.
The 2456 is a festoon-style bulb rated at 12V and designated for specialty applications under the ANSI standard, and it is specified for the K2500 in the 1991, 1998, and 2000 model years.
A festoon bulb seats by pressing its end caps into spring clips rather than inserting into a socket, making it physically incompatible with bayonet or wedge-style housings.
The 2356 is a dual-filament bulb with a BAY15d bayonet base rated at 12V, and it is specified for the 1992 model year. Its dual-filament design and offset base pins distinguish it from the single-filament 1156, and the two are not interchangeable despite both using bayonet-style bases.
Three different bulb base types appear across the K2500 back up light specifications: the BA15s bayonet base, the BAY15d bayonet base, and the wedge base, along with the festoon end-cap mount, indicating that socket compatibility must be confirmed before substituting bulbs across model years.
All four bulb types listed in the specifications conform to ANSI standards applicable in both the United States and Canada.