The thermostat was swapped and it appears to be working as it should now. Time till operating temp is now about 5 mins instead of 7. The old thermostat looked to be stuck open/ stuck in its extended position.

I have both oem coolant temp sensors (top of thermostat housing and bottom corner of radiator) coming in and will see if that addresses the catalyst issue.

 

I'm looking into exhaust leak(s) now. Does anyone know if the 1UZs use exhaust gaskets? Specifically for the input/output of the catalytic converters? Because I'm just seeing them as metal-to-metal.
My Miata has gaskets on both flanges of the converter...different car / different needs though.

 

See the drawing. 17410B and 17451 are gaskets for catalytic converters.

 

Conclusion: The original p0430 code went off by itself and stayed off for about 1,500 miles. The catalyst system FINALLY showed as "complete" and I was able to get it smogged, and it passed.

So I guess changing the post-cat (downstream) O2 sensors with different ones and/ or fixing a vacuum leak on the PCV nipple (the thing sticking out of the valve cover) cleared the issue.

Hopefully this helps someone in the future.

 

Hi, I have the same problem recently. Got exactly the same downstream o2 readings as yours. Did the readings changed back to normal when you replaced the downstream o2 sensors?