If you suspect you have a bad light switch that needs to be replaced, you'll need to cut the power to the switch, test the light switch with a voltage tester to make sure the wires are no longer live, and check the switch itself to see if its continuity or inner wiring and pathways are working correctly. It's a matter of personal safety to ensure the current has been shut off and the wires are safe to touch before you begin removing the switch.
What Is Continuity?
Continuity is the simple term for a closed or complete circuit with a clear path for electrical current to flow properly throughout the circuit. Special testing devices like a continuity tester or multimeter can be used to check continuity.
Before You Begin
At first glance, it can be difficult to tell if a light switch has gone bad. It can be other factors: the light bulb, the power line to the home, or the wiring going to the switch.
First, try the simplest solutions: Make sure the bulb is good, and it is screwed in all the way. Also, ensure the circuit has power and hasn't tripped its circuit breaker or blown a fuse. It's also possible that wire connections going to the switch are loose and not making proper contact. This problem is often corrected simply by shutting off the circuit and retightening the wire connections.
If you have checked all these factors and they check out fine, then it's likely you have a bad switch, especially if the switch is old or the toggle lever feels loose when you snap it. Switches have a limited lifespan, and the springs and other inner components can eventually wear out.
To ensure you're not performing an unnecessary replacement, test the switch for continuity.
Although replacing a switch is relatively easy, there is always a dangerous element to any project that involves handling electricity. You must ensure the circuit has been shut off before you handle the switch or its wire connections.
Also, if you notice visible burns, melted metal, burning smells, or flickering, these could be signs of a short. You can use a multimeter on the continuity or resistance setting to confirm a potential short by checking the voltage reading.
Home wiring systems can take many forms depending on when (and if) the wiring was last updated. If your wiring is very old, even simple projects like replacing a light switch are best handled by a professional.
If you become unsure of what you're doing at any point, stop and call a professional electrician to finish the work.
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Electrical Testing Devices
You have several options for testing a light switch. The simplest, least expensive tool options are a non-contact voltage tester to test the light switch for power before touching the wire connections and a continuity tester for checking the detached switch to evaluate if it is bad.
Another tool, more often used by professionals, is a multimeter, a versatile tool that can be used for various electrical tests. You can use a non-contact circuit tester for a voltage test and a multimeter for testing an on-and-off light switch. The multimeter also has settings to test for power.
- Non-contact voltage tester: A battery-operated device that senses an electrical current without touching the tool's probes to the wires or screw terminals. Held close to live wires, it will light up or buzz (or both) if it senses live current.
- Continuity tester: A simple electrical device with a metal probe, a diode light, and a wire with a clip at one end. It also operates using a battery. This tool tests for continuity—the presence of an uninterrupted electrical pathway. Attached to the screw terminals of a detached switch, the continuity tester lets you determine if the switch lever is properly opening and closing the electrical pathway.
- A multimeter: A versatile battery-powered tester that measures various electrical properties, such as voltage, amperage, and resistance. You can also use a multimeter to test a light switch and do a simple continuity test. To set up a multimeter to test for continuity, turn the tester dial to the "continuity or resistance/ohms" setting. Multimeters are sophisticated tools with many functions. Learning to use them correctly can require some practice.
Make sure you test the testing devices. A battery-powered tester should be tested to ensure the battery is good and the tool functions properly before you use it.
- To test a non-contact voltage tester, hold the tool near an outlet you know has live power and make sure it senses the current.
- 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 0, 0.5, or below.
Equipment / Tools
- Non-contact voltage tester
- Continuity tester or multimeter
- Electrical tape (as needed)
Shut Off the Power to the Circuit
Shut off the power to the light switch circuit by switching off the appropriate circuit breaker in your home's main service panel (breaker box). If you live in an old house with a fuse panel, remove the appropriate fuse by unscrewing it entirely from its socket.
It is very common for the service panel index or circuit labels to be mislabeled, so always check the wires for power before you move on to disconnecting the wires and removing the switch.
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 by touching it with the tip of the tester. 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 small 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.
There is a third type of switch, known as a four-way switch, which is used in conjunction with two three-way switches for circuit configurations where a light fixture (or group of fixtures) needs to be controlled from three or more different wall locations. Four-way switches have four screw terminals, in addition to the green grounding screw. Four-way switches are not common and your home may not have any at all. You generally find four-way switches in very large "open-concept" rooms or in long hallways where you need to control lights from several different entrances.
Disconnect and 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 lever on and off. The tester should light up when the switch lever is in the on position but should not light up when the switch lever is off. If it doesn't exhibit this behavior, it indicates the inner connections are bad and that the switch needs to be replaced.
If you're using a multimeter, set the instrument's dial to the "continuity or resistance/ohms" setting. Touch each tester probe to one of the screw terminals, then turn the switch lever on and off. If the switch is good, the tester will read close to zero when the switch lever is in the on position, indicating that there is perfect continuity (no resistance). When the switch is off, the needle or digital readout should jump to 1 or higher, indicating extreme resistance (no continuity at all).
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. Flip the switch lever to both positions. 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 the switch does not behave in this way, it indicates the switch is bad.
If you're using a multimeter, set the instrument's dial to the continuity or resistance setting, then touch one tester probe to the common (dark-colored) terminal and the other probe to one of the traveler terminals. Turn the switch lever from one position to the other. The tester should read close to 0 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.
Four-way switch: Clip the wire of the continuity tester to one of the darker screw terminals, then touch the other wire to the lighter, brass-colored terminal on the same side of the switch. Flip the toggle lever back and forth: The test should show continuity when the lever is in one position, but not both. Now, repeat the test by touching the second probe to the other brass-colored terminal on the opposite side of the switch. It should show continuity when the lever is in the opposite position from the first test.
Testing with a multimeter is a similar process. With the instrument set to continuity or resistance, touch one probe to one of the darker screw terminals, then the brass-colored terminal on the same side of the switch. The switch should show continuity with the lever in one position but not both. Repeat the test with the other brass screw terminal; its continuity reading should be opposite that of the first brass screw.
If the switch fails any continuity test, it is faulty and must be replaced.
Reconnect (or Replace) the Switch
Connect the switch to the circuit wires to the switch, as before. Tighten each screw terminal and the ground screw securely. If you're replacing the old switch, make sure to 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 strap to the electrical box 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.
Can a light switch just go bad?
Over time, a light switch can go bad, but as a simple device, they don't often fail.
What is the lifespan of a light switch?
A light switch lasts about 30 to 40 years, although a switch used a lot or in a non-temperature-controlled room may have a shorter lifespan.
Can a bad light switch cause a fire?
A switch can't usually cause a fire, but faulty wires within the switch can arc and spark.
Electrical Safety: Safety & Health for Electrical Trades (Student Manual). Electronic Library of Construction Occupational Safety and Health.