O2 sensors measure the burn rate of the exhaust gasses. This information is sent to the computer to calculate the intake of fuel and air for the next combustion stroke. If you have scanned your car and found an O2 sensor code, it doesn't mean the O2 sensor is always faulty. If you use the term 'We are what we eat' then this also applies to your car. If you have bad fuel or cheap 91 fuel in your tank then the combustion burn will no be as high as good 98 fuel. If you look at fuel rates, 91 means only 91% of the fuel will be burnt, another 4 to 5% of the fuel will recirculate and burn up eventually but the remainer 3% will deposit as sludge and gradually build up in your intake and throttle body. If the O2 sensor comes up as faulty, remove the sensor and check for unburnt carbon deposits. cleaning the O2 can save you heaps