P1131 Code

2002 Ford Explorer 4.6L V8, Auto, 4x4, 59500 miles.

I have a code from my computer (P1131), which from what I have found indicates a lean condition. I have removed my front O2 sensors to visually check them for build up, they are clean. I have cleaned and re-installed the MAF, I also replace one suspect hose from my intake tube to a block box looking thing. I have also sprayed down the engine (while running) every place I could see a vacuum line to see if any leaks were prevalent using carb cleaner.

The vehicle starts, idles, and seems to run just fine. I have considered replacing the fuel filter, but I would think I would get the error on both banks and not just one. Any idea’s on where I should look next? Can it just be an O2 sensor?

Also, which side is bank 1 on this engine?

Thanks
Scott