courtesy light

A courtesy light is a small incandescent or LED fixture mounted inside a vehicle cabin, typically in the headliner or door jamb area, that illuminates automatically when a door opens and extinguishes when the door closes. The circuit is controlled by a door switch (usually a plunger switch mounted in the door frame) that completes the ground path when the door moves, allowing current to flow from the battery through the light to ground. Most vehicles have one or more courtesy lights positioned to cast light on the driver or passenger seating area and entry path.

The practical function is simple: to light the interior for safe entry or exit in low-light conditions, and to prevent fumbling for manual switches or trip hazards. In sedans and SUVs, a courtesy light typically draws 5 to 15 watts and remains on only during the seconds around door operation. Some vehicles include a delay circuit that keeps the light on briefly after the door closes, giving occupants time to find the main cabin switch. Pickup trucks often have additional courtesy lights in the bed area or under the rear bumper.

Circuit design and failure modes

The door switch is the critical component. Over time, mechanical wear, corrosion, or debris can prevent reliable contact, causing the light to stay on, remain off, or flicker. Water intrusion into the door cavity can corrode the switch terminals. If the light stays on continuously, the door switch is usually stuck closed, draining the battery; if it never comes on, the switch is likely stuck open or the bulb has failed. Testing requires a multimeter across the switch terminals and visual inspection of the bulb socket for signs of corrosion or loose connections.

The term reflects its purpose rather than its mechanics: the light is a courtesy extended to the driver or passenger, a small convenience feature that became standard in mass-production vehicles starting in the mid-20th century. It is distinct from reading lights (overhead fixtures with a separate switch) and map lights (aimed downward at the lap). Some modern vehicles have replaced bulbs with integrated LED modules sealed into the headliner, eliminating user-replaceable components but also making failure repairs more expensive.

Courtesy lights are wired through the body control module or directly through a dedicated relay in some newer vehicles, allowing engineers to coordinate timing with other door-triggered events such as window operation or lock status updates. The circuit may also include a rheostat or PWM dimmer to match the light level to ambient brightness, reducing glare on night driving. On vehicles with power windows or sunroofs, the courtesy light delay may be extended so occupants can orient themselves after entry.