2000 Nissan Sentra Sensor Bottom of Engine?

I have a 2000 Nissan Sentra with a 1.8L and a manual transmission. There is a plastic plug that looks like a sensor that is on the very bottom of the engine. It has broken off (likely by running over something). My check engine light stays on all the time. I really can't tell that the sensor affects how the car runs. Does anybody know the name of this sensor? I would replace it if I knew. I'm pretty sure it's not the crankshaft position sensor. It's in the wrong place.