1999 Explorer V-6 rough at start-up

My '99 (90K miles) is a bit rough on start up, but settles down to smooth (as can be expected for the pushrod V-6). I have also occasionally gotten a CEL code #171 indication a Left Bank Lean which I take to mean the O2 sensor on that bank needs replacing (which bank is the Left?) Anyone know?

I have had the truck a year, and replaced plugs, wires, PCV, cleaned MAF sensor, etc.

I have also gotten Left & Right Bank Lean CEL messages. The Fix is . . . ?