You will have a total of four oxygen sensors. Two will be before the catalytic converters and two after. I would suggest to rent or buy the special socket to remove them as they are hard to get at on the four wheel drives.
there are 4 o2 sensors
There is a total of 3 O2 sensors 2 Upstream and 1 Downstram
4
4
There are 2 oxygen sensors.
Don't know what your original reading was, but the O2 sensors before the cats moniter how the engine is running and the O2 sensors after the cats moniter how the cats are performing.
2
they have 2 o2 sensors on the muffler ELIA
That would depend on what year of Ford Explorer and which engine you have ( my 1995 Ford Explorer with the 4.0 liter OHV , V6 engine has ( 2 oxygen sensors )
one because its a pre obd2 vehicle
depending on the engine it will have two or three
( two ) I believe they are in the outlets of the two exhaust manifolds