2.5 four cyl. dodge running lean

2.5 four cyl. dodge running lean. failing emissions for NO; coolant not overheating but running hotter than normal. plugs i pulled had white powder look to them. i was told it's running lean by a guy at the parts store by the look of the plugs. I guess a lean mixture would make it run a bit hotter too. Should i suspect the O2 sensor at this point? email snipped-for-privacy@hotmail.com and thanks