Headlights flickering

This week

General guidance for this symptom. A warning light that is flashing or an overheating engine: stop driving and call.

Likely causes

• A bulb at the end of its life

  Halogen filaments weaken before they fail and can flicker with vibration over rough roads. HID bulbs often flicker or change color before they go out.

  Headlight and tail light bulbs
• A loose or corroded connector

  Moisture, dust and heat cycling loosen the plug behind the headlight. Flicker over bumps is the giveaway.

  Headlight and tail light bulbs
• A weak battery connection

  Loose or corroded battery terminals can make all the lights pulse, especially when a big load like the blower or cooling fan switches on.

  Battery check
• A charging system problem

  If the alternator output is unsteady, every light in the car rises and falls with it. A battery light or dim lights at idle often come with it. Alternator repair goes to a repair shop.

  Battery check

Start by noticing which lights flicker. One headlight on its own is local to that light: the bulb, its socket or its plug. If both headlights pulse with the dash lights, especially when the engine idles or the fan kicks on, the problem is upstream in the power supply. That distinction tells you whether it is a quick fix or a charging system test.

One light or all of them?

Aftermarket LED bulbs in housings designed for halogen can flicker because the car's bulb monitoring expects a different load. Your owner's manual lists the bulb type the housing was built for.

What we do in the bay

Our technicians check every exterior light as part of the full-service oil change. For flicker, we look at the bulb and its connector and replace the bulb with the type your car takes. That is the headlight and tail light bulb service, and on many cars it is done right there in the bay.

If both lights pulse with the dash, we run a battery check that includes the charging output with the engine running. If the alternator or the wiring is at fault, that is repair work we do not do. We give you the readings and point you to a repair shop.

A quick test you can do at home

1. Park facing a garage door or wall at night with the engine running and the headlights on.
2. Watch both beams on the wall while someone switches the blower fan and rear defrost on and off.
3. Gently tap the top of each headlight housing. A flicker on the tap points to that light's bulb or plug.
4. Rev the engine slightly. Lights that brighten and dim with engine speed point to the charging side.
5. Note what you saw and tell the technician when you pull in. It saves guessing.

Why it matters on our roads

Winter evenings arrive early, and long stretches of I-580 through Washoe Valley and US-395 south of Carson City are dark between towns. A flickering headlight lights the road unevenly, and if it is the charging system, the battery may be running down while you drive. Federal lighting standards (NHTSA, FMVSS 108) set how much light a headlight must put on the road, and a flickering one falls short of that. Our headlight bulb guide covers the types.