DIAGNOSTICS
Because of the internal circuitry used in a wideband oxygen sensor, you can't hook up a voltmeter or oscilloscope to read the sensor's output directly.  A wideband O2 sensor produces a current signal that varies not only in amplitude but direction.  That makes it quite different from a conventional oxygen sensor that produces a voltage signal that bounces back and forth between 0.1 and 0.9 volts.

The only way you can currently diagnose a wideband oxygen sensor is through the vehicle's on-board diagnostic system using a scan tool.

You can use the scan tool to read the actual air/fuel ratio, and to check the sensor's response to changes that should cause a change in the air/fuel ratio.  Opening the throttle wide, for example, traditionally causes a sudden and brief lean condition followed by a richer mixture as the computer compensates.  But with the new control strategies made possible with wideband O2 sensors, the air/fuel ratio remains steady when the throttle is snapped open.

The diagnostic strategies for wideband O2 sensors vary from one vehicle manufacturer to another but, as a rule, you'll get an oxygen sensor code if the sensor reads out of its normal range, if the readings don't make sense to the computer (should indicate lean when lean conditions exist, etc.) or if the heater circuit fails.

One thing to keep in mind about wideband O2 sensors is that they can be fooled in the same way as a conventional oxygen sensor by air leaks between the exhaust manifold and head, and by misfires that allow unburned oxygen to pass through into the exhaust.  Either will cause the sensor to indicate a false lean condition which, in turn, will cause the computer to make the engine run rich.

OTHER WIDEBAND SENSORS
It's important not to confuse Bosch wideband O2 sensors with those produced by other OEM suppliers.  With some other wideband O2 sensors (such as those used in 1996 and newer Toyotas, for example), a scan tool will display a "simulated" voltage reading between 0 and 1 volt.  The actual voltage output from the sensor is much higher, but the computer is calibrated to divide the sensor's actual output by 5 to comply with OBD II regulations that require a display reading of 0 to 1 volts (these regulations have since been revised to allow the actual voltage to be displayed.)

SENSOR REPLACEMENT
Bosch wideband oxygen sensors are designed for an operational life of 100,000 miles.  Replacement should be needed only if the sensor has failed due to unusual operating conditions, physical damage, or contamination.  Blowing a head gasket can allow silicon to enter the exhaust and contaminate the sensor.  Oil burning can allow phosphorus to enter the exhaust and contaminate the sensor.  If replacement is necessary, use the same type of wideband sensor as the original.