The function of the rear lights in a car
The car taillights are a collective term referring to multiple lighting devices installed at the rear of the vehicle. They work together to convey the vehicle's status, position, and driving intention to vehicles and pedestrians behind, and are a key component for ensuring driving safety.
The car taillight system typically includes the following types of functional lights:
Brake lights (brake indicator lights): When the driver depresses the brake pedal, the brake lights will immediately illuminate, emitting a high-intensity red light, clearly informing the vehicles behind that "I am decelerating or stopping". Their brightness is usually more than three times that of the taillights, with the aim of enabling the following vehicles to recognize and respond instantly, being the core signal for preventing rear-end collisions.
Outline lights (position lights/taillights): They remain illuminated when the vehicle is powered on or during night driving, typically in red or amber color. Their main function is to outline the vehicle's shape and width, allowing vehicles behind to clearly determine the presence and position of the vehicle in low visibility conditions such as rain and fog.
Turn signals: Located on both sides of the taillight group, when the driver activates the turn signal, they will flash in yellow or amber frequency, clearly indicating that the vehicle is about to turn left, right, or change lanes, alerting other road users to avoid.
Reverse lights: They automatically illuminate when in reverse gear, usually in white or red light. Their main function is to illuminate the rear road surface and send a warning signal to the rear pedestrians and other vehicles that "the vehicle is reversing".
High-beam taillights: In extremely low visibility weather conditions such as heavy fog or rainstorms, they are activated, emitting red light with strong penetrating power, significantly improving the vehicle's visibility in adverse weather conditions and preventing collisions due to unclear visibility by following vehicles.
Emergency signal lights (double flash lights): In case of vehicle breakdown, accident, or need for emergency parking, all turn signals can be simultaneously activated, forming a flashing red warning signal to alert surrounding vehicles to avoid.
These lights work together to form the "tail language" of the vehicle, ensuring that other road users can accurately understand the vehicle's dynamics, thereby effectively reducing the incidence of traffic accidents.
Car taillight malfunctions may manifest as either the taillights being constantly illuminated or completely not illuminated, which are two different issues with different causes and troubleshooting methods.
Causes of constantly illuminated taillights
If the taillights remain illuminated after the engine is turned off and the car is locked, this is usually not a normal phenomenon and may be caused by the following reasons:
"Escort me home" function activated: Many models will automatically illuminate the taillights for 30 seconds to 2 minutes after locking the car, facilitating the owner's movement in the dark. This is a normal function and will automatically extinguish after a period of time.
Brake light switch failure: If the switch above the brake pedal is misaligned or the internal contacts are stuck, the brake lights will remain illuminated even if the brake is not depressed. Try gently turning the switch to reset it.
Light control module or body control module (BCM) failure: If the module's program is corrupted or the hardware is damaged, it may cause the lights to fail to turn off normally. Try disconnecting the negative battery terminal for 5 minutes to restart.
Fuse blown: Although a blown fuse usually causes the lights to not illuminate, in some circuit anomalies, it may also cause abnormal constant illumination.
Door or trunk not closed properly: The lighting system of some vehicles will keep the taillights illuminated as a reminder due to the not fully closed doors.
Software or sensor failure: Faults in the vehicle's electronic system software, or incorrect triggering of sensors for the trunk or defrosting function, may also cause the taillights to illuminate abnormally.
Causes of completely non-illuminated taillights
If the taillights are completely not illuminated, common reasons include:
Light bulb failure: This is the most common reason. The taillight bulbs (including brake lights, position lights, turn signals) reach their lifespan or are burned out due to vibration or unstable voltage. Fuse blowout: The fuse in the tail light circuit melts due to short circuit or overload, causing power interruption. Refer to the vehicle manual to find the corresponding fuse and replace it with a new one of the same specification.
Line fault: Includes line breakage, short circuit, loose or corroded plugs, which prevent normal current transmission.
Light switch or relay failure: Poor contact of the switch controlling the tail light or failure of the relay to connect the circuit will cause the tail light not to light up.
Control module failure: Damage to the tail light control module or vehicle body control module (BCM) requires professional diagnostic equipment for detection.
External damage or water ingress: Cracked tail light lens, internal water ingress or impact to the tail light may cause damage to the bulb or circuit board.
Suggested troubleshooting steps
First, wait: After turning off the engine and locking the vehicle, wait for 5 minutes to confirm if it is the "Follow Me Home" function.
Check the bulb: Open the trunk and check if the tail light bulb is blackened, if the filament is broken, and replace it with a bulb of the same model if necessary.
Check the fuse: According to the vehicle manual, find the fuse box in the driver's cabin or engine compartment, and check if the fuse corresponding to the tail light has melted.
Check the connection: Check if the tail light plug is loose, has rust or burnt marks.
Check the brake switch: Step on the brake and observe if the brake light is normally illuminated.
Try to de-energize and restart: Disconnect the negative terminal of the battery for 5 minutes, then reconnect it and reset the vehicle's electronic module.
Seek professional help: If the above steps cannot solve the problem, or if it involves control modules, complex lines, etc., it is recommended to go to a professional repair shop for diagnosis to avoid further damage from disassembling.
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