The most common cause is a vacuum leak. Check that first.
May, 26, 2012 AT 1:28 AM
Never mind the "bad gas" theory. First, it takes a lot for a ford to even set a hard code of which the light in question is an indication. Ford plays funny games with emissions controls to make sure that at least some of their creations can stay on the road for a little while.
Your problem is most likely associated with a vacuum leak (like intake gasket) or possibly messed up O2 sensor.