Mercury Mariner Back Up Light Bulb Size

The Mercury Mariner back up light utilizes 921 and 3156 bulb sizes for the rear illumination system. Data for the specific components covers the period from 2005 through 2011.

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
2007-2011	921
2005-2006	3156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
ANSI standard (US & Canada). Single-filament wedge bulb (T25). 12V, 27W.	3156

Other Locations

Additional Notes

The 921 bulb is the most frequently specified back up light bulb for the Mercury Mariner, covering the 2007-2011 model years.
The 921 is an ANSI-standard wedge bulb rated at 12 volts and 16 watts, classified under the T15 designation, which refers to its tubular wedge form factor measured in eighths of an inch.
The 3156 bulb is specified for the 2005-2006 Mercury Mariner back up light positions.
The 3156 is an ANSI-standard single-filament wedge bulb rated at 12 volts and 27 watts, classified under the T25 designation, making it physically larger than the T15 921.
The 3156 operates at 27 watts, which is 11 watts higher than the 921, meaning the two bulbs draw different amounts of current and are not electrically equivalent.
Both bulb types conform to ANSI standards recognized in the United States and Canada, indicating standardized electrical and physical tolerances within those markets.
The physical size difference between the T15 and T25 form factors means the 921 and 3156 use different socket sizes and are not interchangeable between the 2005-2006 and 2007-2011 model years.
Both bulbs use a wedge-style base, which relies on the bulb glass envelope itself to make contact with the socket rather than a separate metal base pin.