O2 (Oxygen) Sensors are critical to fuel control and to the PCM's converter diagnostics.
The front sensor should be very active, and typically oscillate rapidly from approximately 0 to less than 1 volt. If the front sensor shows low or no voltage, the sensor could be defective; or, there might be an exhaust leak before or immediately behind the sensor.
Typically, the rear sensor should emit a fairly steady signal. If the signal is below 250 mv, check for activity by rapid accelerator kickdown, or by raising the engine speed to approximately 2000 rpm. Some movement should be noted.
Check O2 Sensor operation to ensure vehicle is in fuel control. On most applications, the front O2 Sensor should be switching around 450 mv and the rear O2 Sensor should be above 450 mv, typically 650 - 850 mv. If the rear O2 sensor is not above 450 mv, check the vehicle's emissions with a 4- or 5-gas analyzer. The front AFR sensors should indicate AFR of 14.7 : 1 under most conditions.
Many diagnostic tools exist to accurately test O2 Sensors. Verify operation before replacing any sensors.