The 9004 bulb is an ANSI-standardized halogen type approved for use in the United States and Canada, operating at 12 volts with a dual-filament design rated at 65 watts on the high beam filament and 45 watts on the low beam filament.
The dual-filament construction of the 9004 means a single bulb contains both the high and low beam light sources within one envelope, mounted on a P29t base.
The Volvo 940 uses the 9004 bulb for both the high beam and low beam headlight positions across the 1991-1993 model years, meaning both functions are served by the same bulb type.
Because the high and low beam positions share the same 9004 specification, the two sockets draw from the same bulb design, though each socket activates a different filament within it.
The P29t base is a single-contact prefocus base type, and its specific pin configuration is unique to the 9004 family, which limits interchangeability with other dual-filament halogen types such as the 9007, as those use physically incompatible base designs despite similar voltage ratings.