If one headlight on your car is dim but stable, the odds are that the solution is a simple and easy one. With the effects of aging beginning to impact older cars equipped with high-voltage halogen headlights, this has become an increasingly common problem.
The most likely cause of a dim headlight is a bad bulb. Headlights don't blow out, they dim over time, and rarely do both headlights dim at the same rate.
A bad ground can result in lower than required voltage being delivered to your headlight, which has a particularly pronounced effect on halogen lights.
Bad Ground Causes
This can be caused by corroded bulb terminals, or a loose or broken wire. Consider wire brushing the terminals or replacing the bulb.
Many times, headlight lenses can develop leaks, which can cause condensation to form on the inside of the plastic. This water collects dust, which over time dims the light.
Condensation that dries under the heat of the bulb and can leave deposits or etch the plastic if it contains sulfuric or nitric acid.
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