I take it you mean you are getting a code 52, engine running rich?
Just because the O2 sensor is reading a rich condition does not mean the 02 sensor is at fault, merely that it is receiving a reading outside the parameters. But 02 sensors do wear out.
If your Coolant Temp Sensor(2 wire for the computer, The Sender for the dash gauge is 1 wire) is reading too cold, it will run rich.
A vaccuum leak can cause it to run rich, and other things like the throttle position sensor, Map sensor, failing fuel pressure regulator among others.
All sensors can be tested for resistance, or in the case of the O2 sensor, fluctuating voltage. A CTS can be bought at Rockauto for under 15 dollars.
Haynes or Chilton Manuals tell you how to test the sensors if you have a Digital mulitmeter.
Rockauto also lists O2 sensors from 35 dollars to 95 dollars for an '88 b250