The O2 is changing mV, it's just not changing fast enough. Given a steady dose of fuel (WOT) it will read that since the PCM is commanding a steady fuel constant. Otherwise the PCM is constantly searching for stoich (14.7:1 AFR) by adding fuel until the O2 reads roughly .500 mV, and then subtracting fuel until the O2 reads roughly .400. My explanation isn't to the number, but hopefully it will help you to better understand how things work. I was also told, by someone reputable, that the only way to see if an O2 is actually bad, is to test it with an oscilloscope. That's how quickly it works.
