I finally have a resolution for P0031 and maybe P2097. Went to Mastro Subaru in Tampa,FL and had them replace O2 front a/f sensor. They ended up being good people, didn't charge me for diagnostic, because I kind of knew already what was wrong. So they threw the car on the lift (for free) and checked O2 sensor. Turned out it [sensor] was indeed junk. So I gave them a go-ahead to replace. It came to $319, but I think it's worth it considering I am more than a thousand miles from home.