1998 Mercury Sable O2 sensor

One of your O2 Sensors is easily accessible just popping your hood, just look directly down your exhaust head pipe just below the exhaust manifold and you will see it, that is your forward upstream. Your rear upstream is a little harder to get at, you do need to get underneath the car for it, just behind your transaxle fluid pan you look up and you can see the O2 Sensor, a bugger to get at. And your 2 downstream are relatively easy to get at, they are just behind each catylitic converter, on top of your exhaust pipe. Hope this was of some assistance to you