Well here I go. My first of many questions. My 2005 GTO has 38K miles and I was wondering how often the oxygen sensors need to be replaced, where they are located on the exhaust system and how many there are. Is there one in front and after the cat. converter? Is there a way of checking them and if they need replacing, any good brands from the parts store? I don't need to pay the GM price if I can avoid it.