GMC K1500 Back Up Light Bulb Size

Additional Notes

• The 1156 bulb is the most frequently specified back up light bulb across the GMC K1500 model range, appearing in fourteen of the nineteen model years listed, spanning 1979-1989 and 1993-1997, with the exception of 1990 and 1992.
• The 1156 is a single-filament bulb with a BA15s bayonet base, rated at 12V and 21W. The BA15s designation indicates a single-contact bayonet base with a 15mm collar, which is a common standard in automotive lighting.
• The GMC K1500 uses four distinct back up light bulb types across its model range: the 1156, 2356, 2456, and 3156, none of which are directly interchangeable despite sharing the same 12V operating voltage.
• The 3156 is a single-filament wedge bulb of the T25 form factor, rated at 12V and 27W. It is specified for the 1991 and 1999 model years. Its wedge base differs fundamentally from the bayonet bases found on the 1156 and 2356 bulbs.
• The 2356 is a dual-filament bulb with a BAY15d bayonet base, specified for the 1998 model year. The BAY15d base features two offset pins to prevent incorrect installation, and its dual-filament design distinguishes it from the single-filament 1156 despite a similar bayonet mounting style.
• The 2456 is a festoon-style bulb rated at 12V, specified for the 1990 and 1992 model years. Festoon bulbs are cylindrical in shape and mount by contact pressure at each end rather than using a base socket, making them physically incompatible with bayonet or wedge sockets.
• The shift from the 1156 in 1997 to the 2356 in 1998 represents a change from a single-filament to a dual-filament bulb type, which involves a different base configuration and would not allow substitution between the two.
• All four bulb types listed carry ANSI standard designations applicable to the US and Canadian markets, meaning the numeric codes correspond to standardized specifications for voltage, wattage, and base type within those regions.