Collection: Bulbs T4W

T4W and BA9s: two names for the same bulb

The T4W (alternative designation) and the BA9s (CEE designation) are strictly identical: short cylindrical 4W bulb, 9 mm diameter bayonet base. You can buy either one interchangeably — it's the same product.

BA9s more specifically designates the base: B for bayonet, A for standard shape, 9 for 9 mm diameter, s for single contact (1 pin). T4W more designates the complete format: T for tubular, 4W for power.

Main uses

• Circular dome lights (round) — as opposed to elongated festoon dome lights
• Courtesy lights integrated into sun visors
• Map lights integrated into the headliner
• Some door sill lights
• Dashboard indicator lights (on older vehicles)

T4W vs C5W festoon: how to distinguish them?

• The C5W festoon is elongated (31, 36 or 42 mm) with metal contacts at both ends — it is mounted horizontally in an elongated housing.
• The T4W / BA9s is short and cylindrical with a 9 mm bayonet base (quarter turn insertion + clip) — it is inserted vertically into a circular housing.

Elongated dome light = C5W festoon; round dome light = T4W / BA9s.

Upgrading T4W to LED

The LED T4W upgrade provides a much whiter light than the original yellow halogen. Simple replacement (quarter turn insertion). Modern LED T4W bulbs are generally compatible without a CANbus resistor on most vehicles — dome lights are rarely monitored by the ECU.

Choose the color temperature according to preference: cool white (6,000 K, modern) or warm white (3,500 K, more natural).

See also our BA9s collection (same bulb, different name).