Audi Q3 Fog Light Bulb Size

Additional Notes

• The Audi Q3 rear fog light specifications reference two distinct bulb types across the listed model years: the H21W and the 921.
• The H21W is an ECE-standard bulb used in European and worldwide markets. It operates at 12V and draws 21W, with a single filament and a BAY9s bayonet base, which is a keyed offset-pin base designed to prevent incorrect installation.
• The 921 is an ANSI-standard bulb used in the US and Canadian markets. It is a T15 wedge-type bulb operating at 12V and drawing 16W, with a plastic wedge base that requires no separate socket locking mechanism.
• The H21W and 921 are not interchangeable. The BAY9s bayonet base of the H21W is physically incompatible with the wedge socket used by the 921, and their wattage ratings also differ by 5W.
• The Audi Q3 rear fog light uses the H21W bulb across the 2020-2024 model years based on the listed specifications.
• The 921 bulb is specified for the Q3 rear fog light across the 2015-2018 model years based on the listed specifications.
• The lower wattage of the 921 (16W) compared to the H21W (21W) reflects a difference in the governing standards rather than a difference in intended function, as both serve as rear fog light bulbs in the same vehicle model.
• The BAY9s base of the H21W is an offset bayonet fitting, meaning the two pins are positioned at different heights around the base, which ensures the bulb can only be seated in one orientation within its socket.