Why is the high-mounted brake light on the car constantly on?
The high-mounted brake light of a car remaining on continuously is usually caused by faults in the brake light switch, short circuits in the wiring, damaged bulbs/LEDs, brake system warnings (such as insufficient brake fluid or worn brake pads), or electronic control system failures (such as ECU/ESP issues). It is necessary to troubleshoot in sequence to avoid safety hazards.
Fault Causes and Self-Inspection Steps
When the high-mounted brake light is constantly on, the following issues should be prioritized for inspection. Before proceeding, ensure the vehicle is off and the handbrake is engaged:
Brake light switch failure (accounting for over 40% of faults):
Location: near the brake pedal. There should be a "click" sound when lightly pressing the pedal.
Check: If the light goes out after disconnecting the plug, it indicates that the switch is damaged and needs to be replaced.
Short circuit or poor contact in the wiring:
Focus on checking the tail light wiring connector. If it is oxidized or loose, clean the contacts with WD-40.
Bulb/LED damage:
Open the trunk to inspect the lamp cover. If the filament is broken or the shell is cracked, replace the same model bulb (LED lifespan is approximately 50,000 hours).
Brake system warning triggered:
Turn on the engine compartment to check the brake fluid level (between MIN-MAX marks). If it is below the warning line, add DOT4 fluid and check for leaks.
Inspect the thickness of the brake pads (less than 0.8 cm requires replacement), as excessive wear can trigger sensors.
Electronic control system failure:
If the ABS/ESP fault light turns on simultaneously, use professional equipment to diagnose the ECU module.
Handling Suggestions
Self-Handling Scope: Limited to replacing bulbs, cleaning contacts, or refilling oil. Avoid self-disassembly for other situations.
Repair Indicators:
Short circuit or ECU failure (require multimeter testing).
Brake fluid leakage or excessive wear of the brake pads.
Risk Warning: Continuous lighting may cause battery depletion and mask real brake faults, increasing the risk of rear-end collisions.
Common Faults of the High-Mounted Brake Light of a Car
Bulb failure: This is the most common cause. The high-mounted brake light is used frequently, and bulbs (especially traditional halogen bulbs) are prone to aging or filament melting. LED bulbs have a longer lifespan, but drive circuit faults can also cause the light to fail to turn on.
Brake light switch failure: The switch is located near the brake pedal. If the internal contacts are worn or have poor contact, it cannot send the lighting signal correctly, causing the high-mounted light to fail to turn on.
Wiring issues: Including open circuits, short circuits, loose plugs, or aging wiring harnesses. Continuous vibrations during driving may cause loose connections, which are common but concealed faults.
Fuse blowout: Some models have independent fuses for the high-mounted brake light. If blown, it will cause complete failure.
Body computer or control module failure: For high-end models, the high-mounted brake light is controlled by the body control module (BCM) or braking system module. If the module stores fault codes or has program abnormalities, it may inhibit the lighting operation, requiring a professional diagnostic instrument to read and clear the fault codes.
Lens obstruction or dirtiness: Although not directly causing the light to fail to turn on, severe dirt or foreign objects blocking it will significantly affect the warning effect and require cleaning.
Common Causes of the High-Mounted Brake Light Continuously Illuminating
Brake light switch failure: The internal contacts of the switch are stuck or damaged, sending a continuous "brake" signal, causing the light to remain on.
Short circuit in the wiring: The brake light wiring shorts to the power line, bypassing the switch and directly powering the light, causing it to remain on.
Brake system failure: Low brake fluid level, severely worn brake pads, or brake system leakage. In some models, the brake system warning light will light up and may cause the high-mounted brake light to remain on abnormally. Electronic Control System Failure: The electronic control unit (ECU) of the vehicle or related sensor signals are abnormal, possibly misjudging the vehicle as being in a continuous braking state.
Solutions
Preliminary Self-Check:
Check if the bulb is burned out and replace it with a bulb of the same model.
Check if the fuse is blown.
Clean the lamp cover to ensure there is no obstruction.
Lightly press the brake pedal and feel if the switch is stuck or abnormal.
Professional Diagnosis:
If the self-check is ineffective, it is recommended to go to a professional repair shop or 4S store. The technician will use a multimeter to detect the continuity of the circuit and read the vehicle fault codes through the OBD diagnostic instrument to accurately locate whether it is a switch, module or systemic failure.
Important Reminder: The high-position brake light is an important safety device. Any malfunction should be repaired as soon as possible to ensure that the following vehicle can promptly perceive your braking intention and avoid rear-end accidents.
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