These simple steps should be followed whenever testing O2 sensors.
Scan the vehicle for any O2 sensor codes and analyze the data stream PID. O2 sensor voltage should cycle normally with proper amplitude and frequency. An O2 sensor stuck at a fixed bias voltage is an indication of an open O2 circuit or lack of O2 sensor (dedicated) ground. If possible use a graphing multi-meter to analyze the O2 sensor data to determine any possible problems.
While reading the scan values, goose the throttle and observe for O2 sensor minimum and maximum values (0.1x volts to 0.9x volts). Although this is not a conclusive evidence of correct O2 sensor operation, it serves as a preliminary indication of proper operation.
Some automotive manufacturers employ a dedicated O2 sensor ground wire that is grounded somewhere at the engine block or chassis. A loss or rupture of this ground wire will render the O2 sensor useless. This ground wire feeds only the ECM’s O2 sensor circuit. The main engine ground does not feed this type of O2 sensor circuit.
Verify the O2 sensor wire integrity. Most O2 sensors are biased and an open signal wire will give a reading of whatever the bias voltage is. Later model Jeep/Chrysler O2 circuits tend to be biased at around 2 or 4 volts, therefore, a constant reading of around 2 or 4 volts on a Chrysler is also an indication of an open circuit. In many of these cases, the ECM will put an “O2 sensor High Voltage” code.
Finally, verify for correct O2 sensor operation with a scope or graphing multi-meter. Check for proper amplitude and frequency. Remember that the scanner O2 sensor readings are only interpreted values and may not show the real voltage reading. This is the reason for doing this final manual test.