How many 02 sensors does a 2001 Cadillac deville have

4 total. 1 each manifold. Then 1 before, 1 after the CAT. You should be able to count and trace them from the connections on the top of your engine. Good luck

There are four total. Two upstream (before catalytic converter) and two downstream (after catalytic converter)
The upstream O2 sensors monitor exhaust gases for the peurpose of maintaining correct air/fuel ratio.
The downstream O2 sensors monitor catalytic converter efficiency.