You have 3 o2 sensors on your car,and they are bold on your exauhle system.1 by radiator,1 back to fire wall,1after the cataliy convertor.Good luck.
Where Is A 2000 Mercury Sable Oxygen Sensor Located?
It is on the back of the intake manifold
The O2 sensor behind the exhaust manifold is the one that needs replacing. If your CHECK ENGINE LIGHT comes on, try to have it checked with a OB2 code reader and more than likely it will be in need of replacing on your 2000 Sable. Good Luck :)