A dimming headlight is both annoying and potentially dangerous. If the problem persists, it is best to have the vehicle checked out by a professional, as it could indicate a larger electrical system issue.
A dim headlight has a few possible causes, most involving the vehicle's electrical system. Most of the problems are easy to fix, and if you are comfortable working on your vehicle you can often perform the repairs at home.
It is possible that the solution is simply replacing the bulb in the lamp that is going dim. A bad bulb, or one that is nearing the end of its usefulness, will often dim or flicker. Replacing a bulb is easy to do. Check your vehicle's owner's manual for specific instructions.
Loose wires or corrosion on the bulb contacts can also cause a bulb to dim. Removing the bulb and spraying electrical contact cleaner on the contacts should solve the problem if corrosion is causing the issue. While the bulb is out, check all wires and ensure the wire plug is firmly connected.
Headlights use a lot of amperage, which puts a strain on the car's voltage-supply system. This can cause malfunctions with the headlight wiring's grounding wire, which completes an electrical circuit. The headlight dims if full voltage is not delivered to the filament of the headlight. Cleaning the ground wire or reconnecting to a different ground source should solve the problem.
