I have a 1131 code as well at the moment. At first the CEL was only on occasionally. It would be there one day, gone the next 2 or 3 days, come back, etc.
O2 sensors can go out gradually as they begin responding more slowly. My guess is that the computer has a lower threshold where it stores a code and a higher one where it illuminates the CEL, and mine was right on the border.
