Mercury Mountaineer Back Up Light Bulb Size

The Mercury Mountaineer back up light utilizes bulb sizes 921, 2456, 3156, and 2356 depending on the specific version of the vehicle. Available data for the lighting components covers the range from 1997 through 2010.

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
2007-2010	921
2006	2456
2004-2005	3156
2003	2356
2001-2002	2456
2000	2356
1999	2456
1997-1998	3156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
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). Dual-filament bulb with BAY15d bayonet base. 12V.	2356

Other Locations

Additional Notes

The Mercury Mountaineer backup light specifications span the 1997-2010 model years, with four distinct bulb sizes used across that range: 921, 2456, 3156, and 2356.
The 921 is a T15 wedge-base bulb rated at 12V and 16W. It is the specified backup light bulb for the Mercury Mountaineer across the 2007-2010 model years.
The 3156 is a single-filament T25 wedge-base bulb rated at 12V and 27W. It draws significantly more wattage than the 921, making the two types electrically incompatible as direct substitutes despite both using wedge bases.
The 3156 is specified for the Mercury Mountaineer in 1997-1998 and again in 2004-2005, representing non-consecutive periods of use separated by several model years using different bulb types.
The 2456 is a festoon-style bulb rated at 12V and classified as a specialty application type. Its festoon form factor differs structurally from both the wedge-base 921 and 3156, meaning it is not interchangeable with either of those types.
The 2456 is specified for the Mercury Mountaineer in 1999, 2001-2002, and 2006, making it the backup light bulb for three separate and non-consecutive periods within the model range.
The 2356 is a dual-filament bulb with a BAY15d bayonet base, rated at 12V. Its bayonet mounting base is physically distinct from both wedge and festoon configurations, and its dual-filament design further distinguishes it from the single-filament 3156.
The 2356 is specified for the Mercury Mountaineer in 2000 and 2003, two isolated model years that are not adjacent to each other within the full specification range.
All four bulb types in these specifications conform to ANSI standards applicable in both the United States and Canada, indicating consistent regulatory compliance across the full model year range.
Because the backup light bulb type changes across multiple model years, including between some consecutive years such as 2005 (3156) and 2006 (2456), the specific model year is a necessary reference point when sourcing a replacement bulb for the Mercury Mountaineer.