The Mercedes-Benz E320 brake light utilizes P21/4W and P21W bulb sizes depending on the specific production requirements for the vehicle lighting system. Data for the lighting components covers the range from 1994 through 2003.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz E320 brake light specifications reference two ECE-standard bulb types: P21/4W and P21W. Both operate at 12 volts and share the 21-watt primary filament rating, but they differ in base type and filament count.
The P21/4W is a dual-filament bulb fitted with a BAZ15d bayonet base. Its second filament draws 4 watts, which is the basis for the "/4W" designation. The offset pins of the BAZ15d base prevent incorrect installation orientation.
The P21W is a single-filament bulb fitted with a BA15s bayonet base. The "s" in BA15s denotes a single-contact base with symmetrical pins, meaning the bulb can be seated in either rotational position.
Because the P21/4W uses a BAZ15d base and the P21W uses a BA15s base, the two bulb types are not physically interchangeable. Attempting to fit one in place of the other is not possible without modifying the socket.
For the E320 brake light, the P21/4W was specified for 2000-2003 model years, while the P21W was specified for 1994-1995 and 1997-1999. The 1996 model year returned to the P21/4W.
The Center High Mount Stop Light on the E320 used the P21W across all listed model years, spanning 1994-1999.
Both bulb types conform to ECE regulations, a lighting standard recognized across Europe and numerous other markets worldwide.
The dual-filament nature of the P21/4W makes it suitable for combined stop and tail light functions within a single housing, where the 21-watt filament serves the brake signal and the 4-watt filament serves the running light.