The BMW Z4 brake light utilizes 921, P21/5W, and P21/4W bulb sizes depending on the specific version of the vehicle. Data for the bulb sizes covers the range from 2003 to 2016.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW Z4 brake bulb specifications span model years 2003-2016, with three distinct bulb types appearing across that range: P21/4W, P21/5W, and 921.
The 921 is an ANSI-standard wedge bulb (T15) rated at 12V and 16W, used in the BMW Z4 from 2009-2016. The ANSI standard indicates it was specified for North American markets.
The P21/5W is an ECE-standard dual-filament bulb with a BAY15d bayonet base, rated at 12V with filaments at 21W and 5W. It appears in the 2007-2008 model years and carries the Adaptive designation.
The P21/4W is an ECE-standard dual-filament bulb rated at 12V with filaments at 21W and 4W. Its base type is BAZ15d, which differs from the BAY15d base of the P21/5W, making the two bulbs physically non-interchangeable despite their similar appearance.
The Adaptive designation present on the 921 and P21/5W entries indicates these bulbs function within an adaptive braking light system, which typically modulates light output based on braking intensity.
For the 2006-2008 BMW Z4, the specifications separate brake lighting into Inner Brake and Outer Brake sub-positions, each requiring a different bulb type.
The Inner Brake position for 2006-2008 uses the H21W, a single-filament ECE bulb with a BAY9s base rated at 12V and 21W. The BAY9s base is specific to this bulb type and is not shared with the other single-filament bulbs in these specifications.
The Outer Brake position for 2006-2008 uses the P21W, a single-filament ECE bulb with a BA15s bayonet base rated at 12V and 21W. The BA15s base is a common single-contact bayonet fitting widely used in automotive lighting.
The P21/4W and P21/5W are both dual-filament ECE bulbs of similar wattage on their primary filament, but their secondary filament ratings differ (4W versus 5W) and their base types are not the same, meaning one cannot substitute for the other.
All bulb types listed across the Z4 brake specifications operate at 12V, consistent with standard 12-volt automotive electrical systems.