If you find the light switch won't turn off immediately after you've installed it, it's probably because you wired it wrong, but if the switch has been working and suddenly you find the light stays on when the switch is off, the switch is malfunctioning. The problem is probably that the toggle has disconnected from the circuit terminals.
There's no reason to try to fix a bad light switch. A new switch costs less than $5 in most cases and installing it is one of the easiest home electrical procedures there is. Until you get the time to do it, though, you might have to flip off the breaker to avoid wasting energy.
Light Switch Won't Turn Off After Installation
Although the principle of light switch wiring is fairly basic, things can sometimes get confusing when the switch is in the middle of a circuit or it controls more than one light and there are multiple wires in the electrical box. If the switch won't turn off the lights, it's probably because you bypassed it somehow. Here are some things to look for when you remove the cover plate to inspect the wiring:
- All the white neutral wires should be spliced together and capped. None should be connected to the switch.
- All the ground wires in the box should be clamped or twisted together and connected to the green ground screw on the switch. None of them should be connected to either of the brass terminals.
- All the live wires should be connected to one brass terminal — usually the top one — and all the load wires to the other.
The most likely error is that the wire going to the light that stays on is contacting one of the live wires. To check, disconnect all the wires from each other, turn on the breaker and use a voltage tester to identify the live circuit wire.
Other wires connected to it are probably supplying power to outlets and the light fixture wire may be mixed in. To check, touch each wire in turn to the circuit wire, wearing rubber gloves for safety. When the light goes on, you've found the wire that needs to be on the lower terminal instead of the top one.
Light Switch Only Turns Off in Middle Position
If the switch is just plain worn out, the toggle may disconnect the circuit only when it's in the middle position. This indicates that the switch is faulty, and if you don't replace it, things will only get worse. This situation is potentially dangerous because it could result in arcing between loose terminals inside the switch, and arcing can cause a fire.
How to Test a Switch
If there are a lot of wires in the electrical box and you're not sure whether the malfunction is due to a wiring problem or a faulty switch, you can use a voltage tester to test the switch. You need a tester with two leads, not a non-contact one, for this test.
To perform the test, you need to have the power on, but you don't need to pull the switch out of the wall. Wear rubber gloves for safety, just in case your hand slips.
- Turn the switch on, then touch the tester leads to both of the brass terminals on the switch and verify that the tester LED goes on.
- Turn the switch off, then repeat the test. If the LED goes off, the switch is good and the wiring is faulty, but if the LED stays on, the switch is bad and needs to be replaced.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.