Cadillac ATS Fog Light Bulb Size

Cadillac ATS rear fog light bulb size specifications include 921 and 7444NA bulb options for this vehicle model. The replacement chart covers 2013 through 2018 production years with corresponding bulb size data.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Rear Fog Light

Years	Bulb Size
2017-2018	921
2013-2016	7444NA

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Wedge bulb (T15). 12V, 16W.	921
ANSI standard (US & Canada). Dual-filament wedge bulb (T20), natural amber glass. 12V, 21W/5W.	7444NA

Other Locations

Additional Notes

The Cadillac ATS rear fog light specifications reference two distinct bulb types across its model years: the 921 and the 7444NA.
The 921 is a single-filament wedge bulb classified under the T15 form factor, rated at 12 volts and 16 watts, and carries ANSI standardization for the US and Canada markets.
The 7444NA is a dual-filament wedge bulb under the T20 form factor, rated at 12 volts with a 21W/5W filament configuration, and features natural amber glass rather than a clear envelope.
The dual-filament design of the 7444NA means it serves two lighting functions within a single bulb, with the higher 21W filament and the lower 5W filament operating independently depending on the circuit activated.
The natural amber glass of the 7444NA produces its color through the glass itself rather than through a coated or painted surface, which is a construction distinction relevant to long-term color consistency.
The Cadillac ATS (2013-2016) rear fog light position calls for the 7444NA, while the 2017-2018 model years use the 921 instead.
The T15 and T20 designations refer to the tubular bulb body width, with T20 being physically larger than T15, meaning the two bulb types are not interchangeable by form factor alone.
The wattage difference between the 921 at 16W and the primary filament of the 7444NA at 21W reflects a meaningful difference in power draw, which is relevant to socket and wiring compatibility when considering substitutions.