The function of the car's rear taillights
The main function of the car taillights is to convey the vehicle's status information to vehicles and pedestrians behind, improving visibility during night and in bad weather conditions, thereby ensuring driving safety.
Signal indication function: Different colors of lights convey vehicle dynamics, such as the brake light (red) indicating deceleration, the turn signal (yellow) indicating turning, the reversing light (white) indicating reversing, and the rear fog light (red) enhancing warning in low visibility conditions.
Warning and visibility: In low visibility conditions such as at night, in fog, and in rain, the taillights make the vehicle outline more obvious, alerting the rear vehicles of the distance and position, and reducing the risk of rear-end collisions.
Lighting assistance: The reversing light can illuminate the rear road surface to assist the driver in reversing; some taillights can also provide certain lighting for the rear vehicles.
Compliance with regulations and aesthetics: Taillights must comply with national safety standards (such as GB 7258) to ensure the vehicle is legally on the road; at the same time, their design also enhances the vehicle's appearance recognition and aesthetic appeal.
Common faults of rear taillights mainly manifest as the lights not turning on, constantly lighting up, or physical damage. They need to be diagnosed based on specific phenomena by checking the bulb, wiring, or control module.
Fault phenomena and common causes
Taillights not turning on: Mostly caused by burned-out bulbs, blown fuses, poor contact in the wiring, or switch failures. When both sides do not light up, it often involves the main circuit or the main switch.
Taillights constantly lighting up: May be due to the accidental touch of the light switch, brake switch failure, not closing the door tightly, or abnormal BCM control module. Some models have a delayed extinguishing function after locking the car.
Physical damage and water ingress: Cracked lamp covers and aging sealing rubber strips leading to water ingress may cause short circuits or damage to internal components. Some models have a cracking risk due to material defects.
Troubleshooting steps and solution suggestions
Self-check handling:
Check if the light switch is in the correct position and confirm that the doors and trunk are closed. Check if the bulb has burn marks and replace the same specification fuse or bulb. For minor water ingress, ventilate and dry it; for severe water ingress, remove the lamp cover for cleaning.
Professional repair: If it involves circuit short circuits, control module failures, or cracked lamp covers, it is recommended to go to a professional repair shop for testing to avoid new faults caused by self-operation.
Common causes of constantly lit rear taillights in cars
Brake system abnormalities
Circuit short circuits
Incorrect lighting operation
Involving bulb, switch, and control module components, they need to be diagnosed or sent for repair based on specific phenomena.
Common operational errors and setting checks
Light switch not reset: Not turning off the taillight switch when parking or turning off the engine, or the light control lever being mistakenly set to the parking light position (marked with "P" or "⊢"). This will cause single-sided or double-sided taillights to constantly light up.
Door and handbrake status: Not fully closing the door may trigger the anti-theft system, causing the taillights to light up. The handbrake not being released or the brake pedal being stuck by the floor mat will also cause the brake lights to remain on.
Special function influence: For some models, the "Guide Me Home" function or the activation of the rear trunk opening and rear windshield defrosting function may mistakenly trigger the taillights to light up. Check the central control settings or wait for the function to automatically turn off.
Brake system hardware faults
Brake light switch damage: The switch located above the brake pedal if the internal contacts are stuck, the spring fails, or is jammed. Even when releasing the brake, the circuit remains connected, which is a common fault in older vehicles.
Brake system abnormalities
Insufficient brake fluid, excessive wear of the brake pads, or non-return springs not returning, will cause insufficient system pressure or mechanical jamming, triggering the taillights to constantly light up to alert the driver.
Bulb and relay problems: Bulb filament sticking or damage to the driving board, and cracked taillight relay, may cause the circuit to remain in conduction, resulting in the taillights not being able to be extinguished.
Circuit and Control Module Malfunction
Short Circuit or Grounding: The insulation layer of the tail light circuit is damaged, aged, or damaged by external force, resulting in a short circuit. Abnormal current conduction will cause the tail light to remain on continuously. It is necessary to check if the wiring harness is exposed.
Control Module Failure: The vehicle's body control module (BCM) has a program error, hardware damage, or sensor failure. It may misjudge the light signal, and it is necessary to reset or repair through a dedicated diagnostic instrument.
Battery and System Software: The vehicle's battery is low on power or the electronic control system has a software error, which may also cause some functions to fail, including the inability to turn off the tail light. It is recommended to restart the system or check the battery status.
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