CEL code 51

I am getting code 51 on my 1996 Neon. The engine seems to be sluggish and the gas mileage is terrible. I am getting black smoke when I step on the gas hard. I am thinking either the front O2 sensor, or the MAP sensor. I have pulled the MAP sensor and it was all oiled up. I cleaned it and reinstalled, and reset the CEL. All was fine for 20 miles, now the CEL is back on. Any ideas?

I am getting code 51 on my 1996 Neon. The engine seems to be sluggish and the gas mileage is terrible. I am getting black smoke when I step on the gas hard. I am thinking either the front O2 sensor, or the MAP sensor. I have pulled the MAP sensor and it was all oiled up. I cleaned it and reinstalled, and reset the CEL. All was fine for 20 miles, now the CEL is back on. Any ideas?More than likely its your upstream O2 sensor.