By Gerry Shih SAN FRANCISCO (Reuters) – Apple Inc has offered to replace faulty on-off buttons on the iPhone 5, a rare glitch that it said on Friday affected a small percentage of the previous-generation smartphones. Apple said on its user-support page that iPhone 5 models manufactured through March 2013 may be affected by this issue, in which the button, also known as a sleep/wake mechanism, stops functioning or works only intermittently. Owners can type in their iPhone serial numbers on Apples website to see if their phones qualify for a fix, then either take their gadgets to a store or mail them in to be repaired. Apple will offer the service free of charge to iPhone 5 customers with models that exhibit this issue and have a qualifying serial number, spokeswoman Teresa Brewer said in a statement on Friday.