When driving at dusk the ambient light can fade slowly. In some vehicles an ambient light sensor determines when light levels are low. This is read by the body control unit, which turns the headlights on.
Automatic “dipping” headlights use a sensor located at the front of the vehicle to detect light from oncoming vehicles. If the headlights are on, low beam is selected automatically. This prevents oncoming traffic being dazzled by the vehicles high beam.
Delayed “off” allows the headlights to be left on after the engine has stopped and the doors locked. The headlights will automatically extinguish after a period of time, allowing the driver to safely see their way from the vehicle.
On some vehicles the body control unit will turn the headlights off if the headlights are left on after the ignition switch is turned off and the driver’s door is opened.
On other vehicles headlight warning alarms sound if the headlights are left on after the engine has been turned off and a door is opened. This reduces the risk of the headlights draining the battery.
The body control unit illuminates a warning light when an exterior lighting bulb or globe such as a stop, tail or flasher light has failed. To do this, the body control unit compares current flowing through these circuits to values stored in the computers memory. If the current flowing is outside set parameters the warning light is illuminated
The body control unit can be programmed to keep the interior light illuminated after the doors have been closed for a set period of time, to allow the occupants to locate seat belts or insert the ignition key. It will also illuminate the light when the ignition is turned off to allow the occupants to locate door handles and luggage items.