A wall switch is a mechanical device with moving parts that gets thousands of uses over the course of its lifespan, so it's no surprise that switches eventually wear out and go bad. The first sign of trouble is when the switch begins to behave in a fashion that is abnormal in any way. For example, if there's an audible crackle, snap, or popping sound when you flip the lever, then it's pretty clear that your switch is defective and it's time to replace it.
Some other warning signs are a little more subtle, and some are even more obvious. Here are some of the most common signs of trouble.
Temperamental Light Fixture
You've likely had this experience: When you flip the toggle lever on a light switch to the ON position, it takes a moment for the light to go on, or the light fixture flickers momentarily before turning on. This often indicates that the metal parts inside the switch are wearing out and that its contacts are failing to make the connections needed to complete the electrical path and allow the light to illuminate. Sometimes (not always) this symptom occurs at the same time you notice that the toggle lever has loosened and has lost its "snap" when you flip it.
Even if this happens only occasionally, it's still time to replace the switch. It is also possible that the light fixture or switch has loose connections or a short circuit that causes intermittent failure, so if the problem persists after you've replaced the switch, have a look at the light fixture next.
It's not uncommon to see a small spark coming from inside a switch when you turn it off. This is normal in most cases, and it doesn't indicate a problem. It's caused by a load arc that occurs when the electricity jumps between the contacts as they pull away from each other. It stops as soon as the contacts are far enough apart. That said, a large spark or a spark that makes an audible noise can indicate a faulty light switch.
But if the sparking causes smoking, or if you notice scorch marks on the cover plate or switch toggle, then it is definitely time to replace the switch.
Faulty light switches can make a variety of electrical sound effects, but most can be described with one of the following words: clicking, sizzling, popping, or buzzing. Sometimes this can be caused by loose wire connections, so make sure to check these. But more likely, these sounds are warning signs of a worn or defective switch. Time to replace it.
However, if it's a dimmer switch that buzzes, investigate the light bulb first. The dimming effect can make some light bulbs buzz as the filament structure vibrates. Try a different type or brand of bulb, not just a new bulb of the same type. If that doesn't help, take another hard look at the switch.
Switch Is Warm to the Touch
It is natural for a dimmer switch to generate some heat that can be felt when you operate the switch (normally you'll feel it when you turn it off). This occurs because a dimmer operates by dissipating some of the heat of the full current flow in order to dim the illumination of the light fixture. If the switch feels genuinely hot, though, the dimmer may be going bad, requiring replacement.
But standard toggle-type wall switches generally should feel cool to the touch. If you feel warmth in a standard switch, it's a sign that the electrical flow is not proper, which can mean the switch is failing. Another possibility is that the switch is undersized for its application. A switch rated for 15 amps installed on a 20-amp circuit may heat up if it's controlling a number of light fixtures or devices that draw more than 15 amps of power. If so, then you should replace the switch with a properly rated 20-amp switch. A warm switch can also be caused by loose wire connections, so check these. But if you continue to feel warmth in the switch, it's time to replace it.
Switch Doesn't Feel 'Right'
Often your fingers are the best detectors of a faulty light switch. If the switch toggle lever starts behaving differently—losing its "snap" or becoming stiffer than usual—the switch mechanism is worn and should be replaced before there's trouble.