Troubleshoot headlights by identifying the problem, investigating the potential causes and making the necessary repairs. Typically, problems with a single headlight involve issues with only that headlight's bulb, socket or wiring. Problems with both headlights usually correspond to a blown fuse, broken relay or worn-out switch.
One of the most common problems that affects only one headlight is a blown bulb. Check for a blown bulb by buying a replacement bulb and installing it in the socket. If replacing the bulb does not solve the problem, check the bulb socket for corrosion. If the socket is corroded, replacing the entire headlamp assembly usually resolves the problem. Electrical issues can also lead to one headlight not working. Inspect the headlight's wiring harness for faults, and make sure the headlight ignitor's ground wire is securely attached.
A blown fuse is a common cause for both headlights failing to light. Find the headlight fuse on your vehicle's fuse panel, and inspect it for a fault. With a blown fuse, the small wire that connects the two prongs is broken, while a working fuse has a complete wire. After replacing the fuse, turn the headlights on. If the fuse blows again right away, a short exists somewhere in the circuit.
If replacing the fuse does not solve the problem, check the headlight relay. Pull the relay out of its socket, and shake it gently. If it makes no noise, then the relay is working properly. If you hear a rattling sound, the relay is broken and requires replacement. If replacing the relay does not fix the headlights, remove and replace the headlight switch.