Before you do any checking of codes or disconnecting the battery, get off your seat, go out to your car, pop open the gas door and check to see if your gas cap is completely tight. Vehicles with OBDII diagnostic capabilities ('96+ mustangs) monitor Evaporative System Pressure/Vacuum and control it. If you cap is loose it well put on the check engine light. Since you just filled up, then the light came on, that to me is the most likely answer. Even having the cap a little bit loose with cause a System leak.
Keep us posted.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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