1997 Honda Civic The temperature gage raises when on Inters

Hello, The check engine light has come on at the same time the temp gage moves to almost the top when I drive on the Interstate. It does go down at slower speeds. It also maintains high temp on hot days. I have had the radiator looked at by a mechanic and he didnt find anything out of the ordinary with pressure or any leaks. Could it be the O2 sensor? I can smell a rich fuel smell while driving and afterwards...