How many o2 sensors does a 1997 mercury mountaineer have?

There are a total of ( 4 oxygen sensors ) according to the engine wiring

schematic for the 1997 Mercury Mountaineer in my Haynes repair manual

( 2 oxygen sensors are " upstream " sensors used by the PCM to adjust the air / fuel ratio of the engine )

( 2 oxygen sensors are " downstream " sensors / catalyst monitors )