Ford Expedition Map Light Bulb Size

Additional Notes

• The 12961 is a Philips-designated miniature wedge bulb in the T10 form factor, using a W2.1x9.5d base and rated at 12V and 5W. The T10 designation refers to the bulb's tubular shape, while the W2.1x9.5d base describes a wedge-type connection with no separate metal socket required.
• The Ford Expedition uses the 12961 bulb for the map light across the 2003-2018 model years, with the exception of 2005, making it by far the most prevalent bulb type in these specifications.
• The 578 and 579 are both ANSI-standard festoon bulbs rated at 12V and 12W, but they are not interchangeable. The 578 measures 41mm in length while the 579 measures 44mm, and installing the wrong length can result in a poor or failed electrical contact at the end caps.
• The PC579 is an ANSI-standard festoon bulb designed for a printed circuit board socket rather than the conventional end-cap contact mounting used by the 578 and 579. This socket type difference means the PC579 is not a drop-in substitute for standard festoon bulbs even where the physical length may be similar.
• The 1999-2002 model years of the Ford Expedition specify the PC579, which requires a printed circuit board socket housing in the map light assembly. Substituting a conventional festoon bulb in this application would require a compatible socket.
• The 1997-1998 model years specify the 579 festoon bulb, which uses standard end-cap contacts and is rated at 44mm in length, 12V, and 12W.
• The 2005 model year specifies the 578 festoon bulb rather than the 12961 used in surrounding years. The 578 operates at 12W, compared to the 5W rating of the 12961, reflecting a fundamentally different lamp technology and socket type for that year.
• The 12961 wedge bulb and the festoon-type bulbs (578, 579, PC579) use entirely different mounting systems and are not interchangeable with one another regardless of voltage compatibility.