I'm trying to trouble shoot my 2009 Nissan Frontier QR40DE with a P0171. The sensor on bank 2 is all good only bank 1 is giving me trouble. What I don't understand is why the STFT maxed out at 25% then suddenly drops 0% at idle as shown in the second live data snap shot. The first snap shop is with me revving the engine. Whenever it goes to 0% it goes into open loop briefly then goes back to closed loop. Not sure where to start with craziness like this.

That A/F Sensor looks to be over-responding to small inputs and not responding to large ones, and for that to happen on just one bank pretty much excludes a vacuum leak. The VQ40 has 2.2V A/F Sensors and scanning the voltage may give you a better picture than the lambda. With +25% STFT, the code implies the ECM is still seeing a voltage significantly above 2.2V (lean). If that's a real lean condition and the A/F is good, adding supplemental fuel should pretty much instantly drive down the STFT. If it doesn't come down much or at all, or if there's more than a few seconds of lag before it drops, that's a smoking gun for a bad A/F and not an actual lean condition.

I added propane at the air filter box and these are the results I received. It definitely went to a rich condition and it appears the A/F sensor responded appropriately. I don't know how the heaters work on the sensor so perhaps this is a heater condition on the sensor.

Yep, that looks pretty okay. That means you either have an actual lean condition on bank1 only, or something is fooling the sensor. Has to be something pretty odd, there aren't too many things I can think of to cause that without throwing a bunch of codes. Small upstream exhaust leak, low supply voltage to injectors on one bank, obstructed fuel rail on one bank, stuck intake runner on one side. Plugged cats can cause weird stuff, rich or lean kind of depends on whether it's plugging up or coming apart. But that usually throws codes. You can test that by seeing if the rear O2's respond the same on both banks when you add propane. That's all I can think of. I'll post back if anything else occurs to me.

I did a swap on the bank 1 and 2 sensors. Now I get a P0174. So it's definitely the sensor. Thanks for the tips.

Slightly strange behavior, A/F's usually go way off voltage and/or stop responding when they fail. But glad you got it straightened away!