Replacing a Faulty Switch

For our first job, we'll tackle replacing a standard one light switch that isn't working right. This is only for a single switch circuit and does not cover a 3 or 4-way circuit. If it just stopped altogether, check the bulb first - then the fuse or breaker ;). If it's intermittent or causes flickering lights, and again you've checked the bulb first, just pick up a new switch at your local Home Improvement warehouse. Remember to match the color with the wall plate.

Removing the Face Plate

Use a screwdriver to remove the two screws that hold the face plate on (if this is a multi switch plate there will be more screws). Now before you do anything, check for voltage to the circuit. A good tool for this is a Non-Contact Voltage Detector. They provide an audible and visual alert if HVAC (High Voltage Alternating Current) is detected. You can also use your meter but you'll have to make contact with bare wire, which is difficult at times.
To use your meter, place one lead on the ground (usually green) and the other on the hot wire (usually black). Of course, you have to test both black and white because some installation have been done that weren't exactly to code. If no voltage is detected then the breaker is off. Verify this at the breaker box. If voltage was present, locate the breaker and shut it off. Make sure everyone in the house knows not to touch the breaker until you are done.

Removing the Switch

Unscrew the two screws holding the switch in place and pull the switch out a bit. There are a few ways the wire can be attached to the switch. This is determined by the era it was installed, and the type of switch used. You'll likely have to loosen a screw and either pull the wire out or unwrap it to remove it from its terminal.
Older wiring will have a fabric like insulation around it. It's often brittle. Check the actual insulation around the wire as well. If it's brittle, you might seriously consider replacing your wiring as this can be a dangerous fire hazard.

Testing the Defective Switch

If you'd like to test the switch prior to replacing it, you can use your electrical meter's ohm setting. Put a lead on each terminal and observe the meter. Flip the switch and note any changes. If your meter registers no resistance and then infinity when switched, the switch is good. If it registers either in both positions, or fluctuates excessively, the switch is bad.

Installing the New Switch

A standard toggle switch will have 3 terminals. One is usually colored green, this is the ground. If your wiring isn't too old, it will have a ground wire. This provides a backup in case the neutral gets a short. I highly discourage you from getting switches with 'push-in' type connectors. And don't get the cheapest switch the supply warehouse has to offer. The dollar or two you save on each switch isn't worth the headache or reduced safety factor.
Reconnect the switch just like the old one was. Tighten the terminals down nice and snug. Gently press the switch back in place making sure any wires don't come loose. Install the screws that hold the switch to the box. Replace the switch cover being sure not to over tighten it. You want it snug, but you don't want to crack the plate. Now turn the breaker back on and try the switch!