Final answer:
A labscope or GMM can be used to diagnose an O₂ sensor by analyzing its voltage output, while measuring the resistance of the sensor is not a common method. Inspecting the physical condition of the sensor and checking its serial number are not effective ways to diagnose its performance.
Step-by-step explanation:
The O₂ sensor produces a voltage signal that varies based on the amount of oxygen in the exhaust gases. By connecting a labscope or GMM to the O₂ sensor's signal wire, technicians can monitor the voltage waveform and determine if the sensor is operating correctly.
Measuring the resistance of an O₂ sensor is not a common method for diagnosing it, as its output is voltage-based. Inspecting the physical condition of the O₂ sensor can sometimes provide visual clues of damage or contamination, but it is not a reliable way to diagnose its functionality. Checking the O₂ sensor's serial number is also not relevant to diagnosing its performance.