The failure of the car headlight adjuster may be caused by various reasons. Here are some common reasons:

1. Circuit issue: The car headlight adjuster controls the brightness of the headlights through a circuit. If a circuit malfunction occurs, such as an open circuit, short circuit, or melted circuit, it may cause damage to the regulator.

2. Relay fault: The headlight adjuster usually controls the brightness through a relay. If the relay is damaged or malfunctioning, the regulator may not function properly.

3. Power issue: The headlight regulator requires a stable power supply to operate. If there is a problem with the power supply, such as high or low voltage, it may cause damage to the regulator.

4. Control switch failure: The headlight adjuster is usually controlled by the control switch. If the switch itself is damaged or malfunctioning, the regulator may not function properly.

5. Bulb problem: If the headlight bulb used has problems, such as burning out or poor contact, it may bring too much load to the regulator, causing it to be damaged.

6. Temperature issue: The headlight regulator is usually located in the engine compartment and exposed to high temperatures. Long periods of high temperature may lead to aging and failure of internal components of the regulator.

When the headlight adjuster of a car is damaged, it is recommended that you find an experienced car repair expert to inspect and repair it. They can use professional equipment and knowledge to determine the specific cause of the malfunction and provide you with corresponding solutions. Remember to repair and replace damaged regulators in a timely manner to ensure the safe driving of your car.