93 Dodge Shadow 2.2 liter check engine light comes on?

[[email protected]]

I have a 93 dodge shadow 2.2 liter and whenever I drive on the highway for more than 60 miles the check engine light comes on. I have checked and it throws code 51, which means o2 sensor stuck on lean position, and whenever I turn the car off the check engine light goes off. It will come back on faster at a higher altitude 3000 ft. and when its cold outside, around 30-40 degrees, it idles roughly until it warms up for a couple of minutes. I read it can be the o2 sensor or the map sensor causing this. Which or what should I replace?