The BMW 135i brake light utilizes P21W and H21W bulb sizes for the lighting components located at the rear of the vehicle. Data for the specific bulbs covers the range from 2008 through 2013.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW 135i brake lighting system uses two bulb types across its positions: the P21W and the H21W. Both operate at 12V and 21W, and both carry ECE certification, meaning they conform to European and internationally recognized standards for signalling bulbs.
Despite sharing identical voltage and wattage ratings, the P21W and H21W are not interchangeable. The P21W uses a BA15s bayonet base, while the H21W uses a BAY9s base. These two base types have different pin configurations and are physically incompatible with each other's sockets.
The inner brake position on the BMW 135i uses the H21W across all model years from 2008-2013, making it the only position with a fully consistent bulb fitment throughout the entire production span covered by these specifications.
The outer brake position on the BMW 135i uses the P21W across all model years from 2008-2013, with no variation recorded across that period.
The general brake position, listed separately from the inner and outer positions, specifies the P21W with an "Adaptive" designation for 2008-2009 and 2011-2013. The 2010 model year is the sole exception, where the general brake position lists the H21W (Adaptive) instead of the P21W.
The "Adaptive" designation attached to the general brake bulb entries indicates integration with an adaptive braking or lighting system, which may impose additional electrical or compatibility requirements beyond the base bulb specification alone.
The 2010 BMW 135i general brake position specifies the H21W (Adaptive), which differs from every other model year in that position. This means the 2010 model year requires a BAY9s-based bulb in the general brake socket rather than the BA15s-based P21W used in surrounding years.
The BA15s base of the P21W is a single-contact bayonet fitting, a widely used base type in automotive signalling applications. The BAY9s base of the H21W is an offset-pin bayonet fitting, a less common base type that is specific to certain European-market vehicle applications.