Cadillac CTS back up light bulbs use 921 and 3157 sizes, with outer back up lights requiring 921 bulbs. The replacement bulb size chart covers 2003 through 2019 production years.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Cadillac CTS back up light specifications reference two bulb types across its model years: the 921 and the 3157, both of which conform to ANSI standards recognized in the United States and Canada.
The 921 is a single-filament wedge bulb rated at 12 volts and 16 watts, classified under the T15 form factor. The T15 designation refers to the bulb's tubular shape, with the number indicating its width in eighths of an inch.
The 3157 is a dual-filament wedge bulb rated at 12 volts with two wattage levels: 27 watts on the high filament and 7 watts on the low filament. It carries a T25 form factor classification. The dual-filament design allows a single bulb to serve two lighting functions within one housing.
The Cadillac CTS (2016-2019) uses the 921 bulb for the back up light position.
The 2003-2007 CTS uses the 3157 bulb for the back up light position. The 3157 operates at a notably higher wattage on its primary filament compared to the 921, which reflects a difference in light output between the two generations.
The 921 and 3157 are not interchangeable. Beyond their differing wattage ratings, the T15 and T25 wedge base sizes are physically distinct, and substituting one for the other is not supported by these specifications.
The 2017-2019 CTS also carries a separate Outer Back Up Light position, which uses the 921 bulb, the same type specified for the standard back up light on those model years.
The 2017 CTS Outer Back Up Light specification includes a "Lower" qualifier, indicating that the 921 bulb applies specifically to the lower position within that outer assembly.