Q: if the o2 sensor needs to be replaced. should you replace them all if none have ever been replaced at 114000 miles

Normally, if there is a question as to whether the sensors need to be replaced, checking the datastream with a diagnostic scanner will show how well the oxygen sensor is performing. A couple of things to note about oxygen sensors is that they are responsible for monitoring the oxygen levels in the exhaust so that the control module can adjust the amount of fuel, and therefore, will have a direct effect on mileage and performance. Also, most vehicle manufacturers will list an interval for changing the oxygen sensors, and is normally listed in the owners manual. If you feel you could use some help with diagnosing and/or repairing an issue, contact a certified mechanic, like the mobile mechanics at YourMechanic.com.