The Oxygen sensor or sensors are installed downstream in the exhaust system and generate a small voltage signal (usually less than 1 volt) that increases when the air/fuel mixture goes rich and decreases when the air/fuel mixture goes lean. This "voltage message" is then sent to the Engines computer so that the computer can fine tune the mixture adjustment by regulating fuel quantity, ignition advance etc.

                              Knowing what's bad
Typically, a bad O2 sensor will read low voltage (lean), which causes the engine to run too rich. A low reading can be caused by several things:
1) Old age
2) Contamination
      A) engines burn oil
      B) an internal coolant leak
3) A bad wiring connection
4) An ignition or compression problem in the engine
5) Also an O2 sensor ages. It can be sluggish and not react as fast as it once did so it doesn't need to fail completely to be bad.

                              Heater
1) Sensor needs to operates at 617 to 662 degree minimum to produce voltage
2) If heater fails sensor will still operate but computer will not go into closed loop mode until it gets to operating temp

                                 Testing
1) Disconnect OXSensor lead and connect voltmeter
2) Enrich mixture with propane in the intake until engine speed drops
to 200 rpm or by disconnecting vacuum line to fuel pressure regulator and plug vacuum line
3) Voltmeter should rapidly read .9 volts
4) Test lean response by opening a small vacuum leak
5) Voltmeter should rapidly read .2 volts or below
6) To check the sensors dynamic performance reconnect the sensor lead and run the engine at 1500 rpm
7) Sensor output should fluctuate around .5 volts
8) Monitor sensor signal with a scope at 2500 rpm
10) The signal should fluctuate approximately .2 to .8 volts in less than 300 milliseconds

                     Replacement Recommendations