What would cause my 2015 Ford Expedition check engine light to come on?

The Check Engine Light, CEL, in Your 2015 Ford Expedition is there to warn You when something is wrong in Your vehicle's emissions system. It can be something simple like leaving the gas cap too loose or it can be something serious like a bad 02 sensor or a blown head gasket. The first thing You should check is the gas cap.The reason this causes a warning is because the ecm reads the pressure inside the gas tank to make sure none of the fuel vapors are leaking out. If that isnt Your problem the next most likely cause would be a bad 02 sensor. The best way to determine the problem when it is something more serious is to use an ODB2 scanner. You can purchase one online for under $50 and it will tell You exactly what caused the code so You know what to fix.