After doing some extensive reading, it began to come clear how the O2 Sensor performance could be off, yet not be able to see any symptoms (other than lower gas mileage) with the tools I was using. It turns out that the O2 swing cycle shown in the photographs is the swing time of the overall system and not just the O2 sensor itself. To really examine the performance the O2 sensor, for example to determine if it was developing a lean or rich offset, one would have to use a digital volt meter with an averaging feature connected directly to the sensor and look for the 450mV centerline signal in a known, properly performing system centered at the 50% duty cycle. To measure responsiveness, one would have to use a laboratory scope and setup specifically for this measurement as a properly performing O2 sensor has response times in the neighborhood of a few hundreds of a second, not the average 1.8 seconds seen as the overall system response time by an OBDII scanner.
So when do you replace the O2 sensor? The answer turns out to be simple. Per a trusted Toyota service manager's experience and, experiences posted in the Toyota 4x4wire Forum, the O2 sensor needs to be replaced on average every 75,000 miles to maintain optimum gas mileage performance.
