How to Test a Light Switch

Young man installing light switch at home
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When you flip a switch in your home and nothing happens, you have to start thinking about what could be wrong. Try the simplest solutions first: Make sure the bulb is good and is screwed in all the way. Also, make sure the circuit has power and hasn't tipped its circuit breaker or blown its fuse. When one of these issues isn’t the problem, there's a decent chance that you have a bad switch, especially if the switch is old and/or feels a little loose. There's a sure way to test a light switch for failure. It requires removing the switch from the circuit, and the test is slightly different for single-pole (standard) and three-way switches.

Tools for Testing Light Switches

Testing any type of light switch takes a few basic tools. You'll need screwdrivers, usually one flat-head and one Phillips, for removing the switch and disconnecting the switch wires. You also need a non-contact voltage tester to check the switch and wires for power before touching them. And for making the test, you'll need either a continuity tester or a multimeter.

A continuity tester is a simple electrical device with a metal probe, a tester light, and a wire with a clip at one end. All it does it test for continuity or a complete electrical circuit. A circuit that is "open" is broken and has no continuity. A circuit that is "closed" has continuity. A light switch opens and closes a lighting circuit. There is continuity when the switch is on, and there is no continuity when the switch is off. However, if the switch fails, it does not close the circuit, so there is no continuity.

A multimeter is a versatile tester that measures a variety of electrical properties, such as voltage, amperage, and resistance. It can also be used for a simple continuity test. To set up a multimeter to test for continuity, turn the tester dial to the Continuity or Ohms setting.

It's a good idea to test a continuity tester or multimeter prior to testing your light switch. To test a continuity tester, attach the tester clip to the tester's metal probe; the tester should light up. To test a multimeter, set the dial to Continuity (or Ohms) and touch the two tester probes together: You should get a reading near zero, or 0.5 or below.

Tools and Supplies You Will Need

  • Screwdrivers
  • Non-contact voltage tester
  • Electrical tape (as needed)
  • Continuity tester or multimeter

Instructions

Turn Off the Power

Shut off the power to the light switch circuit by switching off the appropriate breaker in your home's service panel (breaker box). If you live in an old house with a fuse panel, remove the appropriate fuse by unscrewing it all the way from its socket.

Test for Power

Remove the screws on the switch cover plate, and pull off the cover plate to expose the switch wiring. Without touching any wires, test each wire in the electrical box with a non-contact voltage tester. Also, test each of the side terminals on the switch. If the tester detects any voltage (lights up or buzzes), return to the service panel and turn off the correct breaker, then retest again until you detect no voltage.

Identify the Switch Type

Remove the switch's mounting screws, and carefully pull the switch out of the electrical box, straightening the wires as you go. Note the number of side terminals on the switch. If there are two side terminals, it is a single-pole switch. Do not count the ground screw, which is usually green-colored and is near the bottom or top of the switch and connects to a bare copper or green ground wire.

If the switch has three terminals (plus a ground screw), it is a three-way switch. In this case, locate the black or dark-colored screw terminal, and label the wire attached to this terminal, using a piece of electrical tape. This is the "common" terminal that brings power to the switch. You must connect the same wire back to the common terminal; the other two terminals (called "travelers") are interchangeable so they don't need to be labeled.

Remove the Switch

Loosen each of the screw terminals and the ground screw, and pull each wire from its terminal. Bring the switch to your work surface for testing.

Test the Switch for Continuity

Single-pole switch: Clip the wire of a continuity tester to one of the screw terminals, and touch the other terminal with the tester probe. Turn the switch on and off. The tester should light up when the switch is on but not light up when the switch is off.

If you're using a multimeter, touch each tester probe to one of the screw terminals, then turn the switch on and off. When the switch is on, the tester should read close to zero; when the switch is off, it should read "1," indicating no continuity.

Three-way switch: Clip the wire of a continuity tester to the common (dark-colored) screw terminal, and touch the tester probe to one of the traveler terminals. Turn the switch on and off. The tester should light up when the switch is in one position but not in the other position. Move the tester probe to the other traveler terminal (leaving the clip on the common) and repeat the same test.

If you're using a multimeter, touch one tester probe to the common (dark-colored) terminal and the other probe to one of the traveler terminals. Turn the switch on and off. The tester should read close to zero in one position and "1" in the other position. Move the second probe to the other traveler terminal (keeping the first probe on the common) and repeat the same test.

If the switch fails any test, it is faulty and must be replaced.

Reconnect (or Replace) the Switch

Connect the switch to the circuit wires, as before. Tighten each screw terminal and the ground screw securely. If you're replacing the old switch, use a new switch with the same voltage and amperage ratings as the original.

Complete the Job

Push the switch back into place, tucking the wires neatly into the box, and secure the switch with its mounting screws. Reinstall the cover plate. Turn the power back on to the circuit by switching on the circuit breaker or reinstalling the fuse. Test the switch for proper operation.