Service engine soon light on 2003 F150
 

Hey guys,
My light came on today, 2 days after I bought the truck with 154000 miles on it. After taking to to Auto Zone, there is still no reasoning behind why the light is on. They told me that it could not read what the problem was. No code means I don't know what to fix! Any ideas as to what I should do next or to what it could be?