Originally Posted by fast*st
Another thing I wonder about is that all the heat shields have been discarded from underneath, is it possible that at cruise throttle, the pipes are cooling off enough to not let the converter burn? After a run, stop and idle and the rear oxygen sensor drops to a flat line, the above graph was part throttle cruise. Normally on my commute I'm running about 1800 rpm 40 or so in 6th gear.
Meh. The shield is more to keep the heat away from grass and underbody than it is to keep heat in. Sounds like the cat is capable of storing oxy based on your post-cruise datalog.
How many miles?
It may be on it's way out but proper driving cycles and fuel quality will prevent the P0420 from repeating.
Also... I've noticed some non-oxy 91 around here. I didn't notice much of a difference but the switch rate threshold for your downstream 02 is calibrated for oxygenated fuel. It's possible that a batch of non-oxy on a worn cat could make that downstream switch high enough for the PCM to set an efficiency code.