It adjusts your fuel curve to correct lean or rich conditions.

 

Safc II cost me 3 bills. I'm still seeing the first version sell for more than that, but you can pick up the first version for about $260.

If you want to know what the afc does, may I suggest performing a search. There is a lot of good detailed info out there.

Finally, take the car selling to a different location, and quick highjacking my thread.