The Porsche 924 brake light utilizes a P21W bulb size for illumination across all recorded vehicle applications. Data for the specific component covers the period from 1977 through 1988.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The P21W is the sole bulb type specified across all listed model years of the Porsche 924 brake lighting system, covering both the standard brake lights and the center high mount stop light.
The P21W is an ECE-standard single-filament bulb rated at 12 volts and 21 watts, designed for signalling applications such as brake lights.
The P21W uses a BA15s bayonet base, which is a single-contact bayonet fitting. The "s" in BA15s denotes a single contact pin, distinguishing it from the dual-contact BA15d variant.
The BA15s base type is not interchangeable with BA15d bulbs. Substituting a dual-contact base into a single-contact socket results in incorrect fitment.
The Porsche 924 (1977-1982) uses the P21W for its brake lights, as does the 1987-1988 model year, indicating a consistent socket specification across the production run covered by these specifications.
The center high mount stop light on the 1987-1988 Porsche 924 also uses the P21W, meaning it shares the same bulb specification as the standard brake lights on those model years.
As an ECE-regulated bulb, the P21W conforms to European lighting standards, which govern output, base tolerances, and filament placement to ensure consistent performance across compliant fixtures.
The single-filament construction of the P21W means it serves one lighting function per bulb, as opposed to dual-filament types such as the P21/5W, which handle two functions from a single unit.