2001 Kia Optima check eng light

Bought salvage(accident) car in May 09. Car had accident in 2008 mostly body and fender work. Car was hardly driven between repairs and my purchase. Took car on a couple of hard test runs prior to purchase and everything seems fine. Car was also state inspected prior to purchase.
After closing of the deal car was low on gas. Added about 8 Gals. Of Shell mid grade fuel. About a 1/8th of a mile later the check eng. Light came on and stayed on. Auto Zone checked the light with probable causes: high or low fuel pressure, leaking fuel injector or regulator, blocked exhaust or air intake sys.(Light was reset, but on few miles later)
Could the fuel have triggered the light? Your comments or a possible cure. Please.