The Jaguar XJ8 brake light uses P21W and P21/5W bulb sizes depending on the specific production version of the vehicle. Data for the lighting components covers the period from 1998 through 2009.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Jaguar XJ8 brake light circuit uses two distinct bulb types across its production span: the P21/5W for 1998-2006 models and the P21W for 2007-2009 models.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V with a 21W primary filament and a 5W secondary filament. It uses a BAY15d bayonet base, which features offset pins to prevent incorrect installation.
The P21W is also an ECE-standard bulb rated at 12V, 21W, but carries a single filament only. Its base is the BA15s bayonet type, which has symmetrical pins unlike the BAY15d.
Because the P21W and P21/5W use physically different bayonet bases, BA15s and BAY15d respectively, the two bulb types are not interchangeable despite sharing the same 21W rating.
The Jaguar XJ8 (2007-2009) shifted from a dual-filament to a single-filament brake bulb, indicating a change in the brake light circuit design between the 2006 and 2007 model years.
The center high mount stop light on the XJ8 uses the 12961, a Philips-designated miniature wedge bulb of the T10 form factor with a W2.1x9.5d base, rated at 12V and 5W.
The 12961 wedge bulb is retained across all listed center high mount stop light applications from 1998-2008, making it the only bulb in these specifications that remains unchanged throughout the majority of the XJ8 production span.
The W2.1x9.5d base of the 12961 is a wedge-type fitting, which differs fundamentally from the bayonet bases used by the P21W and P21/5W, meaning no fitment overlap exists between the center high mount stop light and the main brake light positions.
Both the P21W and P21/5W conform to ECE regulations, which are applied across European and worldwide markets, reflecting the XJ8's international production standards.
The 5W filament in the P21/5W serves a secondary function, typically acting as a tail or position light within the same lamp housing, a function absent in the single-filament P21W used in later XJ8 model years.