Just like anything heavily used, outlets and switches especially in high-traffic areas of the home, eventually need to be replaced. If a plug or switch is worn it may feel loose or fail to perform its appointed function. If you have one of these it most likely needs replacement.
If your switch produces a cracking sound or the lights flicker when you attempt to use the switch then it is in need of replacemnet. If you have an older house (35 years & older) and have primarily two-prong outlets with no three-pronged plugs, the two-pronged ones should to be replaced for safety reasons.
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