I have a 2003 F-150 w/156K miles. The engine light came on briefly and then went off.

The following code came up when I had the diagnostic done at Advance Auto-

P1451 Manufacturer Control Auxillary Emission Controls

Any clues as to what may be occuring or need fixed?? The only sympton I have right now is lower fuel mileage....around 15-16 mpg highway instead of 18.

Thanks!