Lean left bank code?
 

My wife has a 99 4 runner with about 95k miles on it. Recently it started running very rough upon start up and seemed really down on power. After about a week or so it finally set the SES light as lean left bank I believe was the name of the code. A mechanic friend of mine told me the O2 sensor was reading slow so we replaced it and the MPG went from mid 16's to 19. I also took the MAF sensor off and cleaned it with electrical parts cleaner and a Qtip. Today on the way home from work my wife called and said it set the light again......what could be the problem with this 4 runner?