Low Beams Not Working, High Beams Work Fine

Most of the time, when headlights stop working, you need simply replace them. Sometimes, however, the issue is a little more complicated than just a dead bulb. The most common problem will be the vexing issue of one aspect of the light working fine while another doesn't work at all.

Recently, in our Ask a Mechanic forums, 1995scrapiron had this issue with his old GMC pickup truck. The high beams worked fine, but the low beams did not.

My simple answer was to make sure he was checking both lights to make sure neither was working (better safe than sorry about wasting money on needless parts). Otherwise, the most likely issue is a broken headlight switch.

The Headlight Switch

The first thing to do is test that "headlight dimmer switch" (it's official name) to be sure it's working. Most General Motors vehicles use the same switch design, though aesthetically it may look different. The switch is part of the turn signal switch on the steering column on most trucks. Many other vehicles, even if not made by GM, will be similar.

Access to these switches is usually fairly easy and straight forward. Just use a screwdriver to remove the screws holding the steering column cover in place in the driver's compartment of your vehicle.

A wire-piercing probe and multimeter are highly recommended as testing before replacing is a very good idea. These switches can be expensive. These tools are relatively inexpensive and handy to have for many diagnostic purposes, so it's a good idea for the DIY mechanic to have them and know how to use them.

Find the dimmer switch and its wiring harness and first connector. This is usually just a matter of finding the switch (the handle you use to select low or high beams) and tracing its wiring down to the first connector plug. Most of the time, on most makes and models, the headlights will be a separate plug from the controller's other functions (turn signals and so forth). Your vehicle's maintenance and repair manual should have a diagram to show you which set of wires is which.

Headlamp controls usually only have three wires. One for the high beam circuit, one for the low beam circuit, and one for input. Using your vehicle's repair manual, you'll just need the multimeter to test each of these three wires. Start with the input to be sure you're getting power to the switch itself. Then test the low beam (the one most likely to not be working), followed by the high beam.

If the low or high beam does not transmit power when activated, then the switch needs replacement.

For most headlight "high / low" issues, this test procedure will find your problem. Replacement is often just a matter of removing the screws that hold the selector in place, unplugging the wires, and replacing with a new unit.