The BMW X4 brake light requires a P21W bulb size for proper installation and function. Data for the specified component covers the range from 2015 through 2018.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW X4 brake light specifications cover the 2015-2018 model years, with P21W listed as the brake bulb size across all four years in an adaptive configuration.
P21W is an ECE-standard single-filament signalling bulb rated at 12V and 21W, fitted with a BA15s bayonet base, and recognized for use across European and worldwide markets.
H21W shares the same 12V, 21W electrical rating as the P21W but uses a BAY9s base rather than a BA15s base, making the two bulbs physically non-interchangeable despite their identical wattage.
The 2017-2018 BMW X4 specifications distinguish between inner and outer brake positions, with H21W assigned to the inner brake and P21W assigned to the outer brake.
The base type difference between P21W (BA15s) and H21W (BAY9s) is a critical fitment factor for the BMW X4, as substituting one for the other is not possible without a socket mismatch.
The adaptive designation applied to the P21W entries for 2015-2018 indicates the brake system on those model years incorporates adaptive lighting functionality, which may impose additional compatibility requirements beyond standard bulb ratings.
The BA15s base on the P21W is a single-contact bayonet fitting, while the BAY9s base on the H21W is an offset-pin bayonet fitting, and the physical distinction between these two bases prevents accidental cross-installation.
Both P21W and H21W are single-filament bulbs, meaning each serves a dedicated lighting function rather than combining multiple functions such as brake and tail within one bulb.