What would cause both headlights to stop working?
Most likely, a shared electrical fault takes both headlights offline at once—such as a blown fuse, a failed headlight relay, or a poor ground. In newer vehicles, a fault in the headlight control module or body-control module can also cut power to both lamps. If only one bulb is out, the issue is usually the bulb, its socket, or the wiring to that side.
Common causes of a double headlight failure
The most frequent problems involve the power supply or control path that serves both headlights. Review these possibilities first.
Bulbs vs. electrical faults
If both headlights are out, it's unlikely that two bulbs burned out simultaneously. Start by checking the power and wiring rather than assuming bulb failure.
- Blown fuse(s) for the headlight circuit or a shared power feed
- Faulty headlight relay or a failed relay in the under-hood relay box
- Bad ground connection or corroded connector on the headlight circuit
- Damaged wiring or harness that carries power to both headlights
- Issues with the headlight control module or body-control module that powers headlights
- Low system voltage from a failing alternator or weak battery affecting headlight operation
- In some models, a fault in a DRL (daytime running light) circuit or its module that also affects headlights
These categories cover the majority of two-headlight failures. If these components check out, further testing is needed to pinpoint the exact fault.
Diagnostic steps you can perform
To identify the exact cause, follow a logical sequence of checks that start with the simplest, non-invasive tests and move toward component-level verification. The goal is to determine whether power is reaching the headlight sockets and where the interruption occurs.
- Check the fuses in the main fuse box for the headlight circuit and replace any blown fuse with the correct amperage rating; if a fuse repeatedly blows, there may be a short or wiring fault.
- Inspect and test the headlight relays: locate the headlight relay, swap it with a same‑type relay from the box if available, or test for proper operation with a multimeter or test light.
- Examine headlight connectors and grounds: look for loose connections, corrosion, or damaged wiring at the back of the headlights and along the grounding strap to the vehicle chassis.
- Assess battery and alternator health: with the engine running, measure system voltage (typically 13.5–14.5 volts). Low voltage can prevent reliable headlight operation and may indicate charging system problems.
- Test power at the headlight plugs: with the lights on, use a test light or multimeter to verify whether 12V is present at each headlight socket; if no voltage, backtrack to fuses/relays or the control module.
- Consider control-module involvement: if power and fuses are OK but headlights still don’t operate, the issue may lie with the headlight control module or body-control module, which often requires professional diagnostics.
Car owners who complete these checks can usually determine whether the fault is a simple fuse or relay issue or something that requires a professional electrical diagnosis. If you identify a blown fuse or a faulty relay, replacement may resolve the problem; ongoing issues after these steps typically warrant a mechanic’s inspection.
Safety considerations and next steps
Working on vehicle electrical systems can be hazardous. If you’re uncomfortable with testing or cannot locate the fault, stop and consult a qualified mechanic or dealership service department. Driving without functional headlights is unsafe and illegal in many jurisdictions, especially at night.
Summary
When both headlights stop working, the most common causes are a shared electrical fault—such as a blown fuse, a failed relay, a bad ground, or wiring issues—or a fault in the headlight control/module system in modern vehicles. Start with fuse and relay checks, inspect wiring and grounds, and verify power at the headlight plugs. If those steps don’t identify the problem, seek professional diagnostics to assess modules and more complex wiring concerns.
