Subaru WRX STI Back Up Light Bulb Size

Additional Notes

• The Subaru WRX STI uses the 921 bulb size for the back up light across the 2015-2020 model years.
• The 921 is a wedge-type bulb, classified under the T15 form factor, which refers to its tubular shape measuring 15 units in the T-series sizing convention (where each unit equals 1/8 inch).
• The 921 bulb operates at 12 volts and draws 16 watts, which is standard for incandescent reverse lighting applications in passenger vehicles.
• The 921 follows the ANSI standard, meaning its specifications are governed by the American National Standards Institute and apply to bulbs sold in the United States and Canada.
• The wedge base design of the 921 means the bulb is held in place by the socket itself rather than by a separate metal base with pins, a common construction in modern automotive lighting.
• The T15 wedge format of the 921 is a widely used fitment in reverse light applications, and it shares physical compatibility with some other T15-rated bulbs, though wattage and voltage ratings must match for proper function.