The Mercedes-Benz 420SEL uses a P21/5W bulb size for front turn signals and a P21W bulb size for rear turn signals. The replacement chart covers vehicle production spanning 1986 to 1991.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mercedes-Benz 420SEL uses two distinct bulb types across its turn signal positions: the P21/5W for the front and the P21W for the rear, with both specifications remaining unchanged across the 1986-1991 model years.
Both the P21/5W and P21W are ECE-standard bulbs, a classification that reflects compliance with European and international automotive lighting regulations.
The P21/5W is a dual-filament bulb, meaning it contains two separate filaments within a single envelope, rated at 21W and 5W respectively on a 12V circuit.
The P21W is a single-filament bulb rated at 21W on a 12V circuit, dedicated solely to the turn signal function at the rear of the vehicle.
The P21/5W uses a BAY15d bayonet base, which is an offset twin-pin fitting. The asymmetric pin placement of the BAY15d base prevents incorrect insertion, as the two pins sit at different heights.
The P21W uses a BA15s bayonet base, a single-pin fitting that differs structurally from the BAY15d base used by the P21W's front counterpart.
Because the P21/5W and P21W use different base types, BAY15d and BA15s respectively, the two bulbs are not interchangeable with each other despite sharing the same 21W primary wattage rating.