This is 4.0L V6 Nissan Pathfinder.
Originally it threw P0420 code, which is “Bank 1 Catalyst efficiency below normal”, so I replaced catalyst on Bank 1.
Next it threw PA200, which is for faulty upstream oxygen sensor on Bank 1.
I measured what old sensor was showing, and it was running in narrow range of plus/minus 0.1 volts around 0.26V.
It looked like a faulty sensor, so I replaced it, then ran “Relearn Air/Fuel Mix” procedure.
New sensor shows 0.30V during idle on Bank 1, while the one on Bank 2 idles around 0.60V.
It looked strange and as a bad replacement sensor, so I took car for a test drive and while breaking by the engine, both sensors were showing around 0.95V, so they both were able to detect “rich” condition very nicely.
Still, Bank 1 would return to 0.30V at idle, which is definitely “lean”.
Any ideas how to “gently push” engine’s ECU to get to the normal range?