u asked about the 02 sensor...ur o2 sensor like every other part does wear...it can and will operate incorectly without the check engine light coming on becuase although it may not be operating like it should it still registers within the the computers norm...there are good shops out there that can chekc all the emision and fuel sensors on your car and tell u which ones are failed, going to fail, or operating incorrectly...all they do is check what the sensor is reading against what the books say it should be for optimal performance