P1151 OBD II DTC error code

HO2S Bank 2 Sen 1 Insuff. Switching Lean Mean is the DTC code.  Which O2 sensor does this mean?  Is it anything to worry about or was my daughter just following a Diesel truck putting out some smoke?

You want to replace the o2 sensor opposite cyl#1 befor the cat. Yes I  would replace it will save some pennies in gas.

I got this code after loading my explorer up while helping someone move. I got bad gas mileage because of the weight, and it threw the 1151 code. I reset it and did normal highway driving. The light came back and I am getting 22 mpg in my 96 Explorer V6. That is pretty much the peak of my good fuel economy. Why would I still be getting such good gas mileage?
 
Any info would be great.

22 mpg??  I'd die for that.  My daughter gets 14 mpg, maybe 17 if I put Marvel Mystery Oil in.  If it cost 30 bucks for an O2 sensor to get 6 - 8 mpg more that wouldbe great.  By the way, I reset the code and haven't seen it since.