Collection: Signaling (BA15s, BAY15d...)

Bayonet bases: BA15S, BAY15D, BAU15S

The bayonet system is the most common mounting method for automotive signal lights. The name comes from the shape: insert, turn a quarter turn, and the bulb locks into position. The designation code follows a precise logic:

• BA = bayonet
• 15 = base diameter in millimeters
• S = single contact (1 electrical pin, 1 filament)
• D = double contact (2 electrical pins, 1 or 2 filaments)
• U or Y = offset pins to prevent incorrect insertion during mounting

P21W vs P21/5W: the most frequent confusion

Here is the number 1 confusion in signaling: the P21W and P21/5W look similar but are not interchangeable.

• P21W = single-filament 21W bulb, BA15S base. It performs only one function at a time: stop OR turn signal OR reverse light. The single filament is visible in the bulb.
• P21/5W = double-filament 21W + 5W bulb, BAY15D base. It performs two functions simultaneously in the same light: 5W for the parking light + 21W for the stop light. Two distinct filaments are visible.

How to distinguish between the two when purchasing? Look at the base: BA15S has 2 lugs at the same height (symmetrical); BAY15D has 2 laterally offset lugs (asymmetrical) — it is this offset that prevents reversing stop and parking functions.

Amber turn signals: BAU15S and WY21W

For orange turn signals, two bases dominate. The PY21W (BAU15S base) uses the BA15s format but with lugs offset at 30°, which prevents incorrect mounting — important when the bulb has an amber glass. The WY21W is the capless version (T20) with amber glass. Amber color is legally mandatory in Europe for turn signals — a white turn signal is not approved.

Halogen or LED for signal lights

The switch to LED for signal lights offers several advantages: faster response (useful for stop lights), significantly longer lifespan, reduced consumption. But LEDs consume less than an equivalent halogen: the car's computer may interpret this drop as a burned-out bulb and accelerate the turn signal cadence ("hyperflashing" effect).

Solution: choose a canbus LED (with integrated load resistor) or add an external resistor in parallel. On very recent vehicles with CAN-Bus management, a canbus LED is essential to prevent the "lighting anomaly" warning light from illuminating on the dashboard.