Mercury Mountaineer Dome Light Bulb Size

Additional Notes

• The Mercury Mountaineer dome light uses three distinct bulb types across its 1997-2010 model years: the 578 festoon, the 912 wedge, and the 906 miniature wedge.
• The 578 bulb is the most widely specified type, covering the Mercury Mountaineer from 2002-2010, which represents the majority of the model's production span in these specifications.
• The 578 is a festoon-style bulb with a 41mm length and end-cap contacts, operating at 12 volts and 12 watts. Its end-cap design means it seats by pressing lengthwise into a spring-loaded holder rather than inserting into a socket.
• The 912 is a T15 wedge bulb rated at 12 volts and 12 watts. It is noted as a variant of the 921, meaning the two share a physical form factor, though they are not identical in all electrical characteristics.
• The 906 is a T10 miniature wedge bulb rated at 12 volts and 8 watts, making it the lowest-wattage bulb among the three types specified for this vehicle.
• All three bulb types conform to ANSI standards applicable in both the United States and Canada, indicating standardized electrical and physical tolerances across the North American market.
• The 578 and 912 share the same 12-volt, 12-watt rating, but their physical forms are entirely different. The 578 uses end-cap contacts while the 912 uses a wedge base, making them non-interchangeable despite identical electrical ratings.
• The 906 and 912 are both wedge-type bulbs, but the 906 is a T10 and the 912 is a T15. The T-number refers to the bulb's width in eighths of an inch, so these two types differ in physical size and are not interchangeable.
• The 1999-2000 model years share the 906 specification, while the 1997-1998 and 2001 model years use the 912, reflecting that adjacent model years do not necessarily share the same dome light bulb fitment.