OK, another important point is that my fuel economy has dropped over the past months. There is another current thread on this topic, in which I replied that making sure my tires' air pressure was around 45 seemed to make a big difference. But, even with that checked (and the plugs/wires being relatively new, the air filter and throttle body being cleaned, etc), I am still at least 1 mpg under average. So, this recent P0420 code sounds more and more like a bad 02 sensor that has had me running suboptimal and now finally bad enough to throw the code. I know that I need to try and check the voltage cycling to see if the sensors are within spec, but in general, would a drop in fuel economy point to only the front sensor being bad, or could the rear be involved too? From what I understand, the rear sensor is there just to make sure that the cat is restricting flow, and it does not send anything back to the ECM with respect to air/fuel ratios. The front sensor was replaced 4 years ago and only has 20K on it. The rear sensor is stock.

Thanks for the education!