it was 60+ degrees outside today, so I had everything assembled:
- new NGK Iridium spark plugs (it was $2 less than platinum)
- new upstream and downstream catalysts on bank 1 (where I had initially replaced an upstream, then found a broken flange on downstream one)
- 2 new OEM BOSCH air/fuel ratio sensors (upstream)
- 2 new “cheepo” oxygen sensors (downstream)
on the first run, I’m immediately seeing 2-3 more on MPG: used to have 15-16, now getting 18-19 on regular gas, car “purrs” really
keeping my fingers crossed for next 200-250 miles for P2A00 not to reappear 
Final notes on this thread.
Found PCV valve to go “half-dead”: it was rattling, but blowing to it it was letting air go both directions, very different from new one, so I replaced it at $7.
Drove 290 miles after all that repairs and settled MPG to around 17-17.5 per gallon, no SES light !
So, root cause for P2A00 was two-fold likely:
1: massive exhaust leak on the flange from the front catalyst to the back catalyst
2: PCV valve worn to the point where it was creating what ECU considered a vacuum leak
CASE CLOSED
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It’s been an enjoyable read, great job finding the problems and getting them fixed!
It’s sort of amazing pcv valves aren’t a constant source of problems causing engine lean burns and stalling and the like, since they do as part of their function provide a path for a vacuum leak. When they are working properly the vacuum leak is metered to the correct and repeatable amount is all. You’d think their accurate metering function would fail in no time f rom gunk and wear. But whoever designed them seems to have figured out how to make them pretty robust. I’ve only had to replace them a few times in my 40+ years of driveway diy’er repairs on my own vehicles.
@thegreendrag0n I seem to be experiencing the exact issue you faced here. Did your voltage fluctuate accordingly after replacing Catalytic converter instead of being stuck at 0.3 V?
It did not change a bit before and after repair.
The OBD2 scanner app I was using did not properly read the sensor voltage, it took me some effort to find it out.
Nissan’s VQ40DE engine uses wide-band Air/Fuel Ratio Sensors and looks like generic readers do not use proper PIDs for these engines.